Select the letter-cluster that can replace the question mark (?) in the given letter-cluster series.

AGM, EKN, IOO, ? , UWQ

OSP

Explanation:

AGM, EKN, IOO, ? , UWQ

The logic followed here is :

A → (+4) → E → (+4) → I → (+6) → O → (+6) → U

G → (+4) → K → (+4) → O → (+4) → S → (+4) → W 

M → (+1) → M → (+1) → O → (+1) → P → (+14) → Q

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