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

KGB, JFA, IEZ, HDY, ?

GCX

Explanation:

KGB, JFA, IEZ, HDY, ?

The logic followed here is :

K → (-1) → J → (-1) → I → (-1) → H → (-1) → G

G → (-1) → F → (-1) → E → (-1) → D → (-1) → C

B → (-1) → A → (-1) → Z → (-1) → Y → (-1) → X

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