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

DBK, GDL, JFM, MHN, ?

PJO

Explanation:

DBK, GDL, JFM, MHN, ?

The logic followed here is :

D → (+3) → G → (+3) → J → (+3) → M → (+3) → P 

B → (+2) → D → (+2) → F → (+2) → H → (+2) → J 

K → (+1) → L → (+1) → M → (+1) → N → (+1) → O

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