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

IDK, LFL, OHM, RJN, ?

ULO

Explanation:

IDK, LFL, OHM, RJN, ?

The logic followed here is :

I → (+3) → L → (+3) → O → (+3) → R → (+3) → U  

D → (+2) → F → (+2) → H → (+2) → J → (+2) → L  

K → (+1) → L → (+1) → M → (+1) → N → (+1) → O 

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