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

AEI, EIO, IOU, ?

OUA

Explanation:

AEI, EIO, IOU, ?

The logic followed here is :

A → (+4) → E → (+4) → I → (+6) → O 

E → (+4) → I → (+6) → O → (+6) → U

I → (+6) → O → (+6) → U → (+6) → A 

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