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

MU, NE, PX, QF, SA, TG, ?

VD

Explanation:

MU, NE, PX, QF, SA, TG, ?

The logic followed here is :

M → (+1) → N → (+2) → P → (+1) → Q → (+2) → S → (+1) → T→ (+2) → V

U → (+10) → E → (-7) → X → (+8) → F → (-5) → A → (+6) → G→ (-3) → D

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