Four words are EQUATION, MATHS, WINTER and RAIN. When counted from the left, if every fourth letter in each of the above words is changed to the previous letter in the English alphabetical order, how many words thus formed will have no letter appearing more than once?

​Given:

Four words: EQUATION, MATHS, WINTER, RAIN.

Every fourth letter from the left in each word is changed to the previous letter in the English alphabet.

​EQUATION → EQUZTION (A → Z) - No repeats

MATHS → MATGS (H → G) - No repeats

WINTER → WINSER (T → S) - No repeats

RAIN → RAIM (N → M) - No repeats

All four words (EQUZTION, MATGS, WINSER, RAIM) have no repeated letters.

Thus, correct option is (b).