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

KOY, JPX, IQW, HRV, ?

GSU

Explanation:

KOY, JPX, IQW, HRV, ?

The logic followed here is :

K → (-1) → J → (-1) → I → (-1) → H → (-1) → G

O → (+1) → P → (+1) → Q → (+1) → R → (+1) → S 

Y → (-1) → X → (-1) → W → (-1) → V → (-1) → U

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