In a certain code language, ‘SILK’ is written as ‘TIML’ and ‘INSECT’ is written as ‘IOTEDU’. How will ‘CHANGE’ be written in that language?

Correct — C. Rule: every consonant moves one place forward, every vowel stays exactly where it is.

Test it on both given codes.
SILK: S+1 = T, I is a vowel and holds, L+1 = M, K+1 = L, giving TIML.
INSECT: I holds, N+1 = O, S+1 = T, E holds, C+1 = D, T+1 = U, giving IOTEDU.

Now CHANGE, letter by letter and in order:
C+1 = D, H+1 = I, A holds as A, N+1 = O, G+1 = H, E holds as E.

That spells DIAOHE, option (c).

Why the others are wrong

• (a)
  DEIHAO — It ends in O, but the last letter of CHANGE is the vowel E, which this code never touches. The six letters are right, the order is not.
• (b)
  DOIHAE — Its second letter is O. Position two comes from H, and H+1 = I, so an O there means the shift was taken from the wrong letter.
• (d)
  DEIOHA — Its third letter is I. Position three is the vowel A, which the code leaves untouched, so that slot can only hold A.

This is a substitution code with an exemption, and the exemption is the whole question. Once you notice that the I in SILK and both the E and the I in INSECT come through unchanged, the rule writes itself.

The trap is built into the options. All four are anagrams of DIAOHE — the same six letters in four different orders — so getting the shift right earns nothing unless every letter also stays in its own position.

Work through the word once, left to right, and do not reshuffle at the end.

Key facts

• The vowels A, E, I, O and U are left unchanged by this code.
• Every consonant advances exactly one place in the alphabet.
• All four options are anagrams of the same six letters, so only the order separates them.

Common traps

• Shifting the vowels too, which turns CHANGE into DIBOHF and matches no option.
• Getting all six letters right and then picking the option that has them in a different order.

No directly related past PYQ was found.