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

UKS, ZPX, EUC, JZH, ?

OEM

Explanation:

UKS, ZPX, EUC, JZH, ?

The logic followed here is :

N → (+5) → Z → (+5) → E → (+5) → J → (+5) → O

K → (+5) → P → (+5) → U → (+5) → Z → (+5) → E 

S → (+5) → X → (+5) → C → (+5) → H → (+5) → M

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