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

MN, NM, OL, PK, ?

QJ

Explanation:

MN, NM, OL, PK, ?

The logic followed here is :

M → (+1) → N → (+1) → O → (+1) → P → (+1) → Q

N → (-1) → M → (-1) → L → (-1) → K → (-1) → J

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