Based on the English alphabetical order, three of the following four letter-clusters are alike in a certain way and thus form a group. Which letter-cluster DOES NOT belong to that group?
(Note: The odd one out is not based on the number of consonants/vowels or their position in the letter-cluster.)​

​Logic : 1st + 8 = 2nd, 2nd - 5 = 3rd

a) PXR

P +8 = X, X −5 = S ≠ R
b) LTO

L +8 = T, T −5 = O

c) NVQ

N +8 = V, V −5 = Q
d) HPK

H +8 = P, P −5 = K

So PXR is odd one out. 

Thus, correct option is (a).