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

AR, CP, ?, GL, IJ, KH

EN

Explanation:

AR, CP, ?, GL, IJ, KH

The logic followed here is :

A → (+2) → C → (+2) → E → (+2) → G → (+2) → I

R → (-2) → P → (-2) → N → (-2) → L → (-2) → J

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