Select the term that will come next in the following series.

BDf, GIk, LNp, _________.

QSU

Explanation:

BDf, GIk, LNp, _________.

The logic followed here is :

B → (+5) → G → (+5) → L → (+5) → Q

D → (+5) → I → (+5) → N → (+5) → S 

F → (+5) → K → (+5) → P → (+5) → U 

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