If CUSTOM is written as UCTSMO, then how will PARENT be written in the same…

This is a pair-swap coding pattern: each word is split into adjacent two-letter pairs (1st-2nd, 3rd-4th, 5th-6th), and the two letters within every pair are swapped in place — the pair's position in the word never changes, only the two letters inside it exchange places.

Applying this rule to the given example, CUSTOM, confirms it:

1. Split CUSTOM into pairs: (C, U), (S, T), (O, M).

2. Swap the two letters within each pair: (U, C), (T, S), (M, O).

3. Joining the swapped pairs gives UCTSMO — exactly the code given in the question, confirming the rule.

Now apply the same rule to PARENT:

1. Split PARENT into pairs: (P, A), (R, E), (N, T).

2. Swap the two letters within each pair: (A, P), (E, R), (T, N).

3. Joining the swapped pairs gives APERTN.

Cross-check: applying the same pair-swap rule to APERTN — swap (A, P) back to (P, A), (E, R) back to (R, E), (T, N) back to (N, T) — returns PARENT, confirming the code is consistent and reversible.

Hence, the correct option is APERTN.