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

KRO, PWT, UBY, ?

ZGD

Explanation:

KRO, PWT, UBY, ?

The logic followed here is :

K → (+5) → P → (+5) → U → (+5) → Z

R → (+5) → W → (+5) → B → (+5) → G

O → (+5) → T → (+5) → Y → (+5) → D 

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