Select the Letter Cluster that can replace the question mark (?) in the following series.

MOND, RUUL, WABT, BGIB, ?

GMPJ

Explanation:

MOND, RUUL, WABT, BGIB, ?

The logic followed here is :

M → (+5) → R → (+5) → W → (+5) → B → (+5) → G  

O → (+6) → U → (+6) → A → (+6) → G → (+6) → M  

N → (+7) → U → (+7) → B → (+7) → I → (+7) → P 

D → (+8) → L → (+8) → T → (+8) → B → (+8) → J 

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