If MONKEY is written as XDJMNL, then how is TIGER written in that code?

Correct — B.
Rule: reverse the word, then move every letter one place back (−1).

MONKEY reversed is YEKNOM.
Each −1: Y → X, E → D, K → J, N → M, O → N, M → L = XDJMNL, as given.

TIGER reversed is REGIT.
Each −1: R → Q, E → D, G → F, I → H, T → S = QDFHS → option (b).

Why the others are wrong

• (a)
  ZMHDR — It starts with Z. The code's first letter comes from TIGER's last letter, R, moved back one: Q. It also ends in R, where T − 1 = S belongs.
• (c)
  ZMHFS — It ends correctly in S but opens with Z instead of Q. Reversal puts R first, and R − 1 = Q.
• (d)
  ZMHDS — It opens with Z, not Q, and has D in fourth place. After reversal the fourth letter is I, and I − 1 = H.

When the code letters look unrelated to the word, test a reversal. Write the word backwards under its code and compare letter by letter.

Here every letter of YEKNOM sits one place above the matching code letter, so the rule is reverse, then −1.

Key facts

• MONKEY → XDJMNL is the word reversed (YEKNOM), then each letter moved back one.
• TIGER reversed is REGIT, and REGIT moved back one letter is QDFHS.

Common traps

• Looking for a letter-by-letter shift from MONKEY to XDJMNL without reversing first: M → X and O → D need different shifts.

No directly related past PYQ was found.