Select the letter-pair that can replace the question mark (?) in the following series.

RD, XJ, DP, JV, ?

PB

Explanation:

RD, XJ, DP, JV, ?

The logic followed here is :

R → (+6) → X → (+6) → D → (+6) → J → (+6) → P  

D → (+6) → J → (+6) → P → (+6) → V → (+6) → B  

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