If 'SUN' is coded as 'RTM', what is the code for 'MOON'?

Correct — B.
Rule: every letter moves one place back (−1).

SUN → RTM: S → R, U → T, N → M

MOON: M → L, O → N, O → N, N → M
Code: LNNM → option (b)

Why the others are wrong

• (a)
  LNNP — LNNP codes the last N as P, two places forward. The rule moves every letter one place back, so N becomes M.
• (c)
  LNNO — LNNO codes the last N as O, one place forward. The rule runs backwards, so N becomes M.
• (d)
  LMPM — LMPM gives the two O's different codes, M and P. A fixed −1 shift must turn both O's into N.

A letter-shift code applies one fixed move to every letter. Find the move from the given pair, check it on each letter, then apply it.

Here S → R, U → T and N → M are all −1, so the code is a backward shift of one.

A fixed shift also means repeated letters get repeated codes: the double O in MOON must come out as NN. That alone removes LMPM.

Key facts

• SUN → RTM is a −1 shift: S 19 → R 18, U 21 → T 20, N 14 → M 13.
• Under a fixed shift, the same letter always gets the same code.
• MOON under −1 gives L, N, N, M.

Common traps

• Checking only the first letter: all four options start with L, so M → L decides nothing.
• Shifting forward instead of back once you see the code letters sit next to the originals.

No directly related past PYQ was found.