Which letter cluster will replace the question mark (?) to complete the given series?

CLPM, FNQN, ?, LRSP, OTTQ

IPRO

Explanation:

CLPM, FNQN, ?, LRSP, OTTQ

The logic followed here is :

C → (+3) → F → (+3) → I → (+3) → L → (+3) → O

L → (+2) → N → (+2) → P → (+2) → R → (+2) → T

P → (+1) → Q → (+1) → R → (+1) → S → (+1) → T

M → (+1) → N → (+1) → O → (+1) → P → (+1) → Q

So, the letter-cluster that replaces the (?) to complete the series is "IPRO"