Based on the English alphabetical order, three of the following four letter-cluster pairs are alike in a certain way and thus form a group. Which letter-cluster pair 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 + 8 = 3rd letter and 2nd letter + 10 = 4th letter

Now, we check each options.

Option (a): DK-LU (Follow)​

D + 8 = L, K + 10 = U

Option (b): FM-NU (Not Follow)

F + 8 = N, M + 10 $$\neq$$ U​

Option (c): GN-OX (Follow)​

G + 8 = O, N + 10 = X

Option (d): BI-JS (Follow)​

B + 8 = J, I + 10 = S

Thus, correct option is (b).