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

KQG, JTK, HXO, ECS, ?

AIW

Explanation:

KQG, JTK, HXO, ECS, ?

The logic followed here is :

K → (-1) → J → (-2) → H → (-3) → E → (-4) → A

Q → (+3) → T → (+4) → X → (+5) → C → (+6) → I

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

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