Four letter-clusters have been given out of which three

Solution

The pattern followed here is: The 2nd letter is the immediately preceding letter of the 1st letter and the 3rd letter is the tenth succeeding letter of the 2nd letter as per the English alphabetical series. In option 'A', 1st letter = S, 2nd letter = S - 1 = R, 3rd letter = R + 10 = B In option 'B', 1st letter = L, 2nd letter = L - 1 = K, 3rd letter = K + 10 = U In option 'C', 1st letter = E, 2nd letter = E - 1 = D ≠ F, 3rd letter = D + 10 = N In option 'D', 1st letter = O, 2nd letter = O - 1 = N, 3rd letter = N + 10 = X