Which of the following letter cluster will replace the question mark (?) to complete the given series?

KUCJ, NRFG, QOID, ?

TLLA

Explanation:

KUCJ, NRFG, QOID, ?

The logic followed here is :

K → (+3) → N → (+3) → Q → (+3) → T 

U → (-3) → R → (-3) → O → (-3) → L 

C → (+3) → F → (+3) → I → (+3) → L 

J → (-3) → G → (-3) → D → (-3) → A 

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