What should come in place of the question mark (?) to complete the following letter cluster series?

FHG, IKJ, LNM, ?

OQP

Explanation:

FHG, IKJ, LNM, ?

The logic followed here is :

F → (+3) → I → (+3) → L → (+3) → O 

H → (+3) → K → (+3) → N → (+3) → Q 

G → (+3) → J → (+3) → M → (+3) → P 

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