Four letter-clusters have been given out of which three

Solution

The pattern followed here is as follows: The 2nd letter is the 2nd succeeding letter of 1st letter, the 3rd letter is the 2nd succeeding letter of the 2rd letter and the 4th letter is the 2nd succeeding letter of the 3rd letter as per the English alphabetical series. In option 'a': SUWA → S + 2 = U, U + 2 = W, W + 2 = Y ≠ A In option 'b': KMOQ → K + 2 = M, M + 2 = O, O + 2 = Q In option 'c': EGIK → E + 2 = G, G + 2 = I, I + 2 = K In option 'd': ZBDF → Z + 2 = B, B + 2 = D. D + 2 = F