If in a certain code, 'APPLE' is written as 'ELPPA', how will 'PPLEA' be written?

Correct — A.
Rule: the word is written backwards.

APPLE read from the end: E, L, P, P, A = ELPPA.
PPLEA read from the end: A, E, L, P, P = AELPP → option (a).

Check: a reversal only reorders letters, so the code must use P, P, L, E and A. AELPP does.

Why the others are wrong

• (b)
  LHDHT — LHDHT contains no P, E or A. A reversal only rearranges PPLEA's own letters, so a code with new letters is out at a glance.
• (c)
  TUCES — TUCES keeps just one letter of PPLEA, the E. A reversal keeps all five letters, so four new letters rule it out.
• (d)
  CJXWV — CJXWV shares no letter with PPLEA at all. Reversing a word cannot bring in letters it never had.

Some codes rearrange letters instead of shifting them. Tell which by comparing the letters: APPLE and ELPPA use exactly the same five, so nothing moved along the alphabet, only the order changed.

The order here is full reversal, last letter first. PPLEA is APPLE with its A moved to the end, so its code is ELPPA with the A moved to the front: AELPP.

Key facts

• APPLE reversed is ELPPA.
• PPLEA reversed is AELPP.
• A pure rearrangement code uses the same letters as the word it codes.

Common traps

• Hunting for a letter shift when the code uses exactly the same letters as the word
• Reversing APPLE again instead of PPLEA, the word the question actually asks about

No directly related past PYQ was found.