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

AZA, EVW, ? , OLM, UFG

IRS

Explanation:

AZA, EVW, ? , OLM, UFG

The logic followed here is :

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

Z → (-4) → V → (-4) → R → (-6) → L → (-6) → F 

A → (-4) → W → (-4) → S → (-6) → M → (-6) → G

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