Which of the following letter-clusters will replace the question mark (?) in the given series to make it logically complete?

AZX, CBZ, EDB, GFD, ?

IHF

Explanation:

AZX, CBZ, EDB, GFD, ?

The logic followed here is :

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

Z → (+2) → B → (+2) → D → (+2) → F → (+2) → H

X → (+2) → Z → (+2) → B → (+2) → D → (+2) → F

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