If GARDEN is coded as FZQCDM, how is FZQCDM coded?

Correct — B.
Rule: each letter moves one place back: G → F, A → Z, R → Q, D → C, E → D, N → M.

Apply the same −1 to FZQCDM:
F → E, Z → Y, Q → P
C → B, D → C, M → L
So FZQCDM is coded EYPBCL → option (b).

Why the others are wrong

• (a)
  EYPCBJ — EYPCBJ ends in J, but M − 1 is L. It also writes the fourth and fifth letters as C, B instead of B, C.
• (c)
  EYPCBL — EYPCBL swaps the fourth and fifth letters. C − 1 is B and D − 1 is C, so the middle reads B then C, not C then B.
• (d)
  FZQCDN — FZQCDN is not shifted at all: it repeats FZQCD and changes only the last letter, M → N, which is a +1 step.

GARDEN → FZQCDM is a uniform shift: every letter steps back one place. The question then applies that rule to the code itself, so GARDEN ends up two places back.

Watch the wrap-around at the start of the alphabet: A − 1 is Z, and on the second step Z − 1 is Y.

A quick check: GARDEN's second letter is A, and two steps back from A is Y, the second letter of EYPBCL.

Key facts

• GARDEN shifted by −1 is FZQCDM, and shifted by −2 is EYPBCL.
• The alphabet wraps at its ends: A − 1 = Z and Z + 1 = A.

Common traps

• Writing the fourth and fifth letters in the wrong order: C and D become B and C, in that order.
• Shifting forward because the question codes the code, when the rule GARDEN shows is −1.

No directly related past PYQ was found.