Which of the following letter-clusters will replace the question mark (?) in the given series?

HFK, KHM, ? , QLQ, TNS

NJO

Explanation:

HFK, KHM, ? , QLQ, TNS

The logic followed here is :

H → (+3) → K → (+3) → N → (+3) → Q → (+3) → T

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

K → (+2) → M → (+2) → O → (+2) → Q → (+2) → S

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