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

UY, ZD, EI, JN, ?

OS

Explanation:

UY, ZD, EI, JN, ?

The logic followed here is :

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

Y → (+5) → D → (+5) → I → (+5) → N → (+5) → S  

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