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

KP, HM, EJ, BG, ?

YD

Explanation:

KP, HM, EJ, BG, ?

The logic followed here is :

K → (-3) → H → (-3) → E → (-3) → B → (-3) → Y  

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

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