If "CHINA" is written as "FKLQD", how will "JAPAN" be written?

Correct — B.
Rule: every letter moves forward 3 places.

Check on CHINA → FKLQD:
C → F, H → K, I → L, N → Q, A → D ✓

Apply to JAPAN:
J → M, A → D, P → S, A → D, N → Q

JAPAN = MDSDQ → option (b).

Why the others are wrong

• (a)
  LDRCO — LDRCO mixes steps: J → L is +2, A → D +3, P → R +2, A → C +2, N → O +1. The code needs the same +3 at every letter.
• (c)
  KCPBO — KCPBO uses steps of +1, +2, 0, +1, +1. P stays P, which no +3 code allows.
• (d)
  MDTDR — MDTDR gets J and both As right but moves P → T and N → R, which are +4 steps. P + 3 is S and N + 3 is Q.

A letter-shift code moves each letter a fixed number of places. Find the shift by comparing positions in the example: C is 3rd and F is 6th, so +3.

Confirm it on every letter before using it, because some codes change the step from letter to letter. Here all five letters of CHINA move +3, so JAPAN gets the same treatment.

Key facts

• Positions: C 3, F 6, J 10, M 13, P 16, S 19.
• A fixed-shift code keeps repeated letters repeated: both As of JAPAN become D.

Common traps

• Checking the shift on the first letter only and missing a code that changes step
• Miscounting P → S and picking the +4 option MDTDR

No directly related past PYQ was found.