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

AOS, CQU, ? , GUY, IWA

ESW

Explanation:

AOS, CQU, ? , GUY, IWA

The logic followed here is :

A → (+2) → C → (+2) → E → (+2) → G → (+2) → I

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

S → (+2) → U → (+2) → W → (+2) → Y → (+2) → A

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