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

GHB, LMG, PQK, ?

STN

Explanation:

GHB, LMG, PQK, ?

The logic followed here is :

G → (+5) → L → (+4) → P → (+3) → S 

H → (+5) → M → (+4) → Q → (+3) → T 

B → (+5) → G → (+4) → K → (+3) → N 

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