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 - 2 = 3rd letter and 2nd letter - 2 = 4th letter
Now, we check each options.
​Option (a): MO - KM (Follow)

M - 2 = K, O - 2 = M
​Option (b): ​CG - LP (Not Follow)​

C - 2 $$\neq$$​ L, G - 2 $$\neq$$​ P
​Option (c): ​SU - QS (Follow)​

S - 2 = Q, U - 2 = S
​Option (d): ​QS - OQ (Follow)​

Q - 2 = O, S - 2 = Q
Thus, correct option is (b). ​​