Four letter-clusters have been given, out of which, three are alike in some manner and one is different. Select the odd letter-cluster.

The logic followed here is: In the above letter-cluster, 2nd letter is immediately preceding letter of the 1st letter. 3rd letter is 2nd preceding letter of the 2nd letter. 4th letter is 3rd preceding letter of the 3rd letter as per the English Alphabetical series. In option 'a': FECA → F - 1 = E, E - 2 = C, C - 3 = Z ≠ A In option 'b': XWUR→ X - 1 = W, W - 2 = U, U - 3 = R In option 'c': NMKH → N - 1 = M, M - 2 = K, K - 3 = H In option 'd': SRPM → S - 1 = R, R - 2 = P, P - 3 = M Therefore, 'FECA' is the odd letter-cluster.