Four children are sitting in a row. A is occupying the seat next to B but not next to C. If C is not sitting next to D, who is occupying seat adjacent to D ?

Start with the fixed pair. A must sit next to B, so A and B form a block (AB or BA). Test the three places this block can go in four seats.

If the block sits in seats 1-2 or 3-4, the other two seats are next to each other, so C and D would be adjacent. That breaks the rule that C is not next to D, so both cases fail.

So the block sits in seats 2-3. C and D take seats 1 and 4, which are not adjacent, so that is allowed. If A is in seat 2, C cannot be in seat 1 (next to A), so C is in seat 4 and D in seat 1. If A is in seat 3, C must be in seat 1 and D in seat 4. Only two arrangements survive: D A B C and C B A D. In both, D's only neighbour is A.

• Option A (B) is wrong because B is never next to D.
• Option C (C) is wrong because C and D are never adjacent.
• Option D (Impossible to tell) is wrong because the constraints fix the arrangement.

Core concept: linear seating with adjacency and non-adjacency conditions. Lock the block first, then test the remaining seats. This is the same disciplined step-by-step elimination that helps in CSAT-style reasoning and in logical problems in Mains General Studies.

Trap: Aspirants often build one arrangement quickly, such as D A B C, and stop, then pick B by accident. Always check the other valid case, C B A D, before deciding.