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

CBOM, FCQN, ?, LEUP, OFWQ

IDSO

Explanation:

CBOM, FCQN, ?, LEUP, OFWQ

The logic followed here is :

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

B → (+1) → C → (+1) → D → (+1) → E → (+1) → F

O → (+2) → Q → (+2) → S → (+2) → U → (+2) → W

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

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