Which of  the following letter-cluster will replace the question mark(?) in the following series? 

COL, FMO, IKR, ?, OGX​​

Given: COL, FMO, IKR, ?, OGX​​

Logic: first letter increasing by 3, second letter decreasing by 2 and third letter increasing by 3

As,

C+3=F, F+3=I, I+3=L, L+3=O

and

O-2=M, M-2=K, K-2=I, I-2=G

Similarly,

L+3=O, O+3=R, R+3=U, U+3=X

So, Missing term is LIU.

Thus, correct option is (b).