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 letters - 3 = 2nd letters, 2nd letters - 2 = 3rd letters.

Option (a): NKI

N - 3 = K, K - 2 = I

Option (b): AXV

A -3 = X, X - 2 = V

Option (c): VSQ

V - 3 = S, S - 2 = Q

Option (d): CAX

C - 2 = A, A - 3 = X

There are three options A, B, C which are following the same logic.

So, the odd one out is CAX.​

Thus, the correct option is (d) CAX.