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 - 5 = 2nd letter and 2nd letter + 3 = 3rd letter

Now, we check each options.

Option (a): OJN (Not Follow)

O - 5 = J, J + 3 $$\neq$$ N​

Option (b): LGJ (Follow)​

L - 5 = G, G + 3 = J

Option (c): RMP (Follow)​

R - 5 = M, M + 3 = P

Option (d): UPS (Follow)​

U - 5 = P, P + 3 = S

Thus, correct option is (a).