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

GAM, DCJ, AEG, XGD, ?

UIA

Explanation:

GAM, DCJ, AEG, XGD, ?

The logic followed here is :

G → (-3) → D → (-3) → A → (-3) → X → (-3) → U

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

M → (-3) → J → (-3) → G → (-3) → D → (-3) → A

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