All good athletes want to win and all athletes who want to win eat a well-balanced diet; therefore all athletes who do not eat a well-balanced diet are bad athletes. The best conclusion from this statement is that

(a) no bad athlete wants to win.
(b) no athlete who does not eat a well-balanced diet is a good athlete. ✓ UPSC's answer
(c) every athlete who eats a well-balanced diet is a good athlete.
(d) all athletes who want to win are good athletes.

Why the answer is (b)

• Let G be good athletes, W be those who want to win, and D be those who eat a well-balanced diet.

• The premise 'All good athletes want to win' translates to G ⊆ W.

• The premise 'All athletes who want to win eat a well-balanced diet' translates to W ⊆ D.

• Combining these, we get G ⊆ D, meaning all good athletes eat a well-balanced diet.

• The contrapositive of G ⊆ D is: if an athlete does not eat a well-balanced diet (not D), they are not a good athlete (not G).

• This matches option (b): no athlete who does not eat a well-balanced diet is a good athlete.

Why the other options are wrong

(a) no bad athlete wants to win.: The premises do not establish a direct link between being a bad athlete and not wanting to win; bad athletes could still want to win but fail to eat well.
(c) every athlete who eats a well-balanced diet is a good athlete.: Eating a well-balanced diet is a necessary condition for being a good athlete, not a sufficient one; thus, not all who eat well are necessarily good athletes.
(d) all athletes who want to win are good athletes.: While all good athletes want to win, the premises do not state that all who want to win are good athletes; they only state that those who want to win eat well.

Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2015, held on 23 August 2015. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.