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

AAI, AEI, AII, AOI, ?

AUI

Explanation:

AAI, AEI, AII, AOI, ?

The logic followed here is :

A → (0) → A → (0) → A → (0) → A → (0) → A 

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

I → (0) → I → (0) → I → (+3) → I → (0) → I 

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