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

MCN, NCO, OCP, PCQ, ?

QCR

Explanation:

MCN, NCO, OCP, PCQ, ?

The logic followed here is :

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

C → (0) → C → (0) → C → (0) → C → (0) → C 

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

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