In a certain code language, 'BELL' is written as 'MKFA' and 'BOND' is written as 'EMPA'. How will 'BLEW’ be written in that language?

Correct — A.

Rule: reverse the word, then shift the reversed letters +1, -1, +1, -1.

BELL reversed is L L E B, and L+1 = M, L-1 = K, E+1 = F, B-1 = A, giving MKFA.
BOND reversed is D N O B, and D+1 = E, N-1 = M, O+1 = P, B-1 = A, giving EMPA.

BLEW reversed is W E L B.

W+1 = X
E-1 = D
L+1 = M
B-1 = A

The code is XDMA, option (a).

Why the others are wrong

• (b)
  XMDA — It keeps L before E, the order they have in BLEW. The code is built on the reversed word W E L B, in which E comes before L, so D must precede M.
• (c)
  XMAD — It ends A, D. Reversing BLEW puts B last, and B-1 = A, so A has to be the final letter of the code rather than the third.
• (d)
  XDAM — Its last two letters are swapped, coding B before L. In the reversed word W E L B, L sits third and B fourth, which gives M then A.

This is a two-stage letter code: a reversal, then a positional shift.

Read the given pair as a whole rather than letter by letter. B does not become M, but B becomes the last letter A, and the final L becomes the first letter M — that mismatch is the signal to reverse.

Once the reversal is spotted the shift reads off cleanly: letters in the 1st and 3rd places of the reversed word step one forward, letters in the 2nd and 4th step one back.

Two worked pairs are exactly enough to fix both stages, and both should be checked before the rule is used on the third word.

Key facts

• BELL reversed is LLEB, and +1, -1, +1, -1 on those letters gives MKFA.
• BOND reversed is DNOB, and the same alternating shift gives EMPA.
• An alternating shift changes sign with position, so one letter can move forward in one slot and back in another.

Common traps

• Shifting first and reversing afterwards, which lands the right letters in the wrong slots.
• Testing the rule against one given pair only, when both pairs have to agree.

No directly related past PYQ was found.