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

GF, KI, OL, SO, ?

WR

Explanation:

GF, KI, OL, SO, ?

The logic followed here is :

G → (+4) → K → (+4) → O → (+4) → S → (+4) → W

F → (+3) → I → (+3) → L → (+3) → O → (+3) → R

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