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 letter + 7 = 2nd letter and 2nd letter + 7 = 3rd letter

Now, we check each options.

Option (a): NUB (Follow)

N + 7 = U, U + 7 = B

Option (b): VCI (Not Follow)​

V + 7 = C, C + 7 $$\neq$$ I​

Option (c): GNU (Follow)​

G + 7 = N, N + 7 = U

Option (d): IPW (Follow)​

I + 7 = P, P + 7 = W

Thus, correct option is (b).