What is C's surname ?

• From clue 2, C wears a tie. Since each person wears a different garment, A and B wear the jacket and sweater in some order.

• Clue 1 states neither B nor Ribeiro wore a white sweater. This implies B does not wear a white sweater. If B wore the sweater, it would not be white. If B wore the jacket, the sweater is worn by A.

• Clue 4 states Kumar does not wear a jacket. Clue 5 states Ribeiro does not wear black.

• Let's determine who wears the sweater. If B wore the sweater, it cannot be white (Clue 1). If A wore the sweater, B wears the jacket.

• Consider Clue 1: 'neither B nor Ribeiro wore a white sweater'. This means the white sweater is not worn by B and not worn by Ribeiro. Therefore, the white sweater must be worn by the person who is neither B nor Ribeiro. The people are A, B, C. The surnames are Ribeiro, Kumar, Singh.

• Let's map surnames to people. C wears a tie. Clue 3 says Singh's garment was not white. Clue 5 says Ribeiro does not wear black.

• Who wears the white sweater? It is not B (Clue 1) and not Ribeiro (Clue 1). So the white sweater is worn by either A or C, but C wears a tie. So the white sweater is worn by A or B? No, B doesn't wear white sweater. So A or C wears white sweater? C wears tie. So A or B wears sweater. B doesn't wear white sweater. So if A wears sweater, it can be white. If B wears sweater, it is not white.

• Let's look at Clue 1 again: 'neither B nor Ribeiro wore a white sweater'. This implies the white sweater is worn by someone who is not B and not Ribeiro. The candidates for wearing the white sweater are A, B, C. C wears a tie, so C doesn't wear a sweater. So the white sweater is worn by A or B. Since B doesn't wear a white sweater, A must wear the white sweater. Thus, A wears the white sweater.

• Since A wears the white sweater, B wears the jacket (since C wears the tie). So B wears the jacket.

• Now, A wears the white sweater. Clue 1 says Ribeiro did not wear a white sweater. So A is not Ribeiro.

• Clue 4 says Kumar does not wear a jacket. B wears the jacket, so B is not Kumar.

• So B is either Ribeiro or Singh. A is either Kumar or Singh.

• Clue 5 says Ribeiro does not wear black. Clue 3 says Singh's garment was not white. A wears white, so A is not Singh. Therefore, A is Kumar.

• Since A is Kumar, and B is not Kumar, B is either Ribeiro or Singh. C is the remaining surname.

• We know A is Kumar. So surnames left for B and C are Ribeiro and Singh.

• Clue 3: Singh's garment was not white. A wears white, so A is not Singh (consistent). B wears jacket. C wears tie.

• Clue 5: Ribeiro does not wear black.

• Let's check colors. A wears white sweater. Remaining colors: blue, black. Remaining garments: jacket (B), tie (C).

• If B is Ribeiro, B cannot wear black (Clue 5). So B would wear blue. Then C wears black. C's surname would be Singh. Check Clue 3: Singh (C) wears black, which is not white. This works.

• If B is Singh, B wears jacket. Clue 3 says Singh's garment is not white. B wears jacket, color unknown yet. C is Ribeiro. Clue 5 says Ribeiro (C) does not wear black. So C wears blue. Then B wears black. Check Clue 3: Singh (B) wears black, not white. This also seems possible? Wait, let's re-read Clue 1 carefully. 'neither B nor Ribeiro wore a white sweater'. We established A wears white sweater. So A is not Ribeiro. Correct.

• Let's re-evaluate the 'Singh' constraint. Clue 3: Singh's garment was not white. A wears white. So A is not Singh. We deduced A is Kumar. So B and C are Ribeiro and Singh.

• Case 1: B is Ribeiro, C is Singh. B (Ribeiro) wears jacket. Ribeiro doesn't wear black (Clue 5). So B wears blue. C (Singh) wears tie. Remaining color is black. So C wears black. Clue 3: Singh (C) wears black, not white. Valid.

• Case 2: B is Singh, C is Ribeiro. B (Singh) wears jacket. C (Ribeiro) wears tie. Ribeiro doesn't wear black (Clue 5). So C wears blue. Remaining color is black. So B wears black. Clue 3: Singh (B) wears black, not white. Valid.

• Is there a constraint I missed? 'neither B nor Ribeiro wore a white sweater'. In Case 2, B is Singh. Ribeiro is C. C wears tie. So Ribeiro (C) did not wear a white sweater (wore blue tie). B (Singh) did not wear a white sweater (wore black jacket). This holds.

• Wait, look at Clue 1 again. 'neither B nor Ribeiro wore a white sweater'. This phrasing usually implies that the white sweater exists and is worn by someone else. We found A wears white sweater. So A is not B and A is not Ribeiro. So A is Kumar or Singh. We found A is Kumar because A wears white and Singh doesn't wear white. So A is Kumar. This is solid.

• So A = Kumar, White, Sweater.

• B and C are Ribeiro and Singh.

• B wears Jacket. C wears Tie.

• Colors left: Blue, Black.

• Clue 5: Ribeiro does not wear black.

• Clue 3: Singh's garment was not white. (Already satisfied as A is white).

• If B is Ribeiro: B cannot wear black. So B wears Blue. C (Singh) wears Black. C is Singh.

• If B is Singh: B wears Jacket. C is Ribeiro. C cannot wear black. So C wears Blue. B (Singh) wears Black. C is Ribeiro.

• Both cases seem logically valid based on the text provided. However, UPSC keys are definitive. Let's look for a subtle constraint. 'neither B nor Ribeiro wore a white sweater'. Does this imply B and Ribeiro are distinct people? Yes, surnames are unique. If B were Ribeiro, the statement 'neither B nor Ribeiro' would be redundant but true. But surnames are different for each friend. So B is not Ribeiro. So B is either Kumar or Singh. We know A is Kumar. So B is Singh or Ribeiro? No, A is Kumar. So B is Ribeiro or Singh. C is the other.

• Let's check the official key: (a) Ribeiro. This implies C is Ribeiro. This corresponds to Case 2: B is Singh, C is Ribeiro.

• Why is Case 1 (C is Singh) invalid? In Case 1, B is Ribeiro. B wears Blue Jacket. C is Singh. C wears Black Tie. Is there a contradiction? Clue 3: Singh's garment was not white. C wears Black. OK. Clue 5: Ribeiro does not wear black. B wears Blue. OK.

• Let's re-read Clue 1: 'neither B nor Ribeiro wore a white sweater'. If B is Ribeiro, this is impossible because surnames are unique. So B is not Ribeiro. So B is Singh or Kumar. A is Kumar. So B is Singh. Therefore, C is Ribeiro.

• Ah, I missed that A is Kumar. If A is Kumar, then B cannot be Kumar. So B is either Ribeiro or Singh. C is the other. The question is, can B be Ribeiro? If B is Ribeiro, then C is Singh. If B is Singh, then C is Ribeiro.

• Is there a constraint on B? Clue 1 says 'neither B nor Ribeiro...'. This implies B and Ribeiro are two different entities in the context of the exclusion, or simply that the person named B and the person named Ribeiro did not wear it. Since surnames are unique, B is not Ribeiro. So B is either Kumar or Singh. We established A is Kumar. So B is Singh. Therefore, C is Ribeiro.

• Step-by-step: 1. A wears White Sweater (deduced from Clue 1 and 2). 2. A is not Ribeiro (Clue 1). 3. A is not Singh (Clue 3, A wears White). 4. Therefore, A is Kumar. 5. Since A is Kumar, B is not Kumar. 6. B is not Ribeiro (surnames unique, and Clue 1 lists them separately, but more importantly, if B were Ribeiro, the statement 'neither B nor Ribeiro' is just 'B didn't wear it', which is fine, but let's look at the options). Actually, the key deduction is: A is Kumar. Remaining surnames for B and C are Ribeiro and Singh. Clue 1 says 'neither B nor Ribeiro wore a white sweater'. This implies B and Ribeiro are distinct people? Not necessarily, but in logic puzzles, listing two names usually implies they are the subjects. However, the strongest link is: If B were Ribeiro, then B is Ribeiro. If B is Singh, C is Ribeiro. Let's look at Clue 5: Ribeiro does not wear black. If C is Ribeiro, C wears Tie. If B is Ribeiro, B wears Jacket.

• Let's assume the standard interpretation: The set of people is {A, B, C}. The set of surnames is {Ribeiro, Kumar, Singh}. A=Kumar. So {B, C} = {Ribeiro, Singh}. Clue 1: 'neither B nor Ribeiro wore a white sweater'. This is a constraint on the person B and the person Ribeiro. It doesn't force B != Ribeiro directly, but combined with A=Kumar, B is either Ribeiro or Singh. If B is Ribeiro, then C is Singh. If B is Singh, C is Ribeiro. Is there a reason to reject B=Ribeiro?

• Let's check Clue 3: Singh's garment was not white. If C is Singh (Case 1), C wears Tie. Color? If B is Ribeiro (Blue), C is Black. Singh (C) wears Black. Not white. OK.

• Let's check Clue 5: Ribeiro does not wear black. If B is Ribeiro (Blue), OK.

• Why is the answer Ribeiro? Perhaps I made a mistake in 'A is Kumar'.

• Re-evaluate A's surname. A wears White Sweater. Clue 3: Singh's garment not white. So A is not Singh. Clue 1: Ribeiro did not wear white sweater. So A is not Ribeiro. So A is Kumar. This is solid.

• So A = Kumar.

• B and C are Ribeiro and Singh.

• Clue 1: 'neither B nor Ribeiro wore a white sweater'. This implies that the person B and the person Ribeiro are the ones excluded. If B were Ribeiro, the statement is 'B did not wear a white sweater'. If B is not Ribeiro, it's 'B did not... and Ribeiro did not...'.

• Usually, in these puzzles, if the answer is unique, there must be a contradiction in one case.

• Let's look at Clue 4: Kumar does not wear a jacket. A is Kumar, wears Sweater. OK.

• Let's look at Clue 5: Ribeiro does not wear black.

• Let's look at Clue 3: Singh's garment was not white.

• If B is Ribeiro: B wears Jacket. Ribeiro (B) doesn't wear black. So B wears Blue. C is Singh. C wears Tie. Remaining color Black. C wears Black. Singh (C) wears Black. Not white. OK.

• If B is Singh: B wears Jacket. C is Ribeiro. Ribeiro (C) doesn't wear black. So C wears Blue. B wears Black. Singh (B) wears Black. Not white. OK.

• Both seem valid. However, the official key is (a) Ribeiro. This means C is Ribeiro. This corresponds to the case where B is Singh. Why is B=Ribeiro invalid?

• Maybe 'neither B nor Ribeiro' implies B and Ribeiro are different people? If B were Ribeiro, the phrase would typically be 'B (Ribeiro) did not...'. The use of 'nor' suggests two distinct entities. If B and Ribeiro are distinct, then B is not Ribeiro. Since A is Kumar, B must be Singh. Therefore C is Ribeiro. This is the likely logical step intended by the question setter.