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.)

Correct option is A

Let's check the options:
Logic: 1st letter - 5 = 3rd letter, 2nd letter - 4 = 4th letter

Option (A) RT-MO → Odd one out
R - 5 = M, T - 5 = O
Option (B) OQ-JM
O - 5 = J, Q - 4 = M
Option (C) WY-RU
W - 5 = R, Y - 4 = U
Option (D) UW-PS
U - 5 = P, W - 4 = S
All option follow (-5, -4) except option A, making it odd one out.
Thus, the correct option is (A) RT-MO