Which letter-cluster will replace the question mark (?) in the following series ?

SAG, KSY, CKQ, ?

SAG

Explanation:

Explanation:

SAG, KSY, CKQ, ?

The logic followed here is :

S → (-8) → K → (-8) → C → (-8) → U

A → (-8) → S → (-8) → K → (-8) → C

G → (-8) → Y → (-8) → Q → (-8) → I 

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