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

QKR, UOV, YSZ, CWD, ?

GAH

Explanation:

QKR, UOV, YSZ, CWD, ?

The logic followed here is :

Q → (+4) → U → (+4) → Y → (+4) → C → (+4) → G 

K → (+4) → O → (+4) → S → (+4) → W → (+4) → A  

R → (+4) → V → (+4) → Z → (+4) → D → (+4) → H 

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