A is studying in

• From condition (ii), the Arts college is in city S, which is in Rajasthan.

• From condition (v), the Science college is in Kerala.

• From condition (i), D is studying in Assam, so D is not in Rajasthan or Kerala.

• Since A is in the Commerce college (condition iii), A cannot be in the Arts college (Rajasthan) or the Science college (Kerala).

• The remaining states for the Commerce and Engineering colleges are Gujarat and Assam.

• Since D is in Assam, D must be in either the Commerce or Engineering college. However, A is in Commerce. If A were in Assam, D would have to be in Engineering. But we need to determine A's location specifically.

• Let's map the colleges to states: Arts=Rajasthan, Science=Kerala. Remaining: Commerce and Engineering are in Gujarat and Assam.

• D is in Assam. A is in Commerce. If A were in Assam, A would be in Commerce and D would be in Engineering. If A were in Gujarat, A would be in Commerce and D would be in Engineering (in Assam) or Commerce (impossible as A is Commerce). Wait, D is in Assam. The colleges in Assam and Gujarat are Commerce and Engineering.

• A is in Commerce. D is in Assam. D cannot be in Commerce if A is in Commerce (different students). So D is in Engineering (in Assam) or Commerce (in Assam). But A is in Commerce. So the Commerce college is either in Assam or Gujarat.

• If Commerce is in Assam, A is in Assam. Then D (in Assam) must be in Engineering. This is possible.

• If Commerce is in Gujarat, A is in Gujarat. Then D (in Assam) must be in Engineering. This is also possible.

• Let's look at the cities. B is in city Q (condition iv).

• Arts is in S (Rajasthan). Science is in Kerala.

• We need to place B. B is in Q. Q is not S (Arts/Rajasthan). So B is not in Arts.

• B is not in Science (Kerala) necessarily, but let's see.

• Let's re-evaluate the state-college mapping.

• States: Gujarat, Rajasthan, Assam, Kerala.

• Colleges: Arts (Rajasthan), Science (Kerala). Remaining: Commerce, Engineering in Gujarat, Assam.

• Students: A (Commerce), D (Assam).

• If A is in Assam, A is in Commerce (Assam). Then D is in Assam, so D is in Engineering (Assam).

• If A is in Gujarat, A is in Commerce (Gujarat). Then D is in Assam, so D is in Engineering (Assam).

• Is there a constraint on B? B is in city Q.

• Cities: P, Q, R, S. S is Rajasthan (Arts).

• So B is not in S. B is in P, Q, or R.

• Let's look at the options. (a) Rajasthan, (b) Gujarat, (c) city Q, (d) Kerala.

• A is in Commerce. Commerce is in Gujarat or Assam.

• If A is in Assam, the answer would be Assam (not an option).

• Therefore, A must be in Gujarat.

• Why can't A be in Assam? If A is in Assam, the answer 'Assam' is not listed. The options are Rajasthan, Gujarat, City Q, Kerala. Since 'Assam' is not an option, A cannot be in Assam.

• Thus, A must be in the other state for Commerce, which is Gujarat.