In a certain code language, 'SHE' is written as 'EIT' and 'SAD' is written as 'DBT'. How will 'SUN’ be written in that language?

Correct — D. Both codes end in T and both words begin with S, which is the signal that the word is being written backwards.

Rule: write the word in reverse, then leave the first letter of that reversal alone and move the next two one step forward.

SHE → reversed E H S → E, H + 1 = I, S + 1 = T → EIT
SAD → reversed D A S → D, A + 1 = B, S + 1 = T → DBT
SUN → reversed N U S → N, U + 1 = V, S + 1 = T → NVT → option (d)

Why the others are wrong

• (a)
  NYT — NYT keeps N first and T last but puts Y in the middle. Y is U + 4, and this code never moves a letter more than one step, as S → T does in both worked words.
• (b)
  NTY — NTY has plausible letters in the wrong seats. T belongs at the end, because it comes from the S that the reversal sends to the last position.
• (c)
  NTV — NTV shifts correctly and then swaps V and T. Reversing SUN gives N-U-S, so the V from U must come second and the T from S third.

Codes at this level are usually two transformations stacked, not one. Read the letters that did not move first: E stays E in the first pair and D stays D in the second, and in both cases that letter was the last of the word and the first of the code.

That fixes the reversal. Then look at what did change — the other two letters each advanced by one — and the rule is complete.

Testing it on the second given word before applying it to SUN is what separates a solved item from a guessed one.

Key facts

• S + 1 = T, U + 1 = V, A + 1 = B and H + 1 = I in the English alphabetical order.
• The code leaves the word's last letter unchanged and advances the other two by one step.
• Both worked words start with S, which is why both codes end in T.

Common traps

• Shifting all three letters when one of them stays put
• Getting the shifts right but writing the code in the word's own order

No directly related past PYQ was found.