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

QOP OPQ OQP PRQ

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"