Which of the following letter clusters will replace the question mark (?) and complete the following letter cluster series?

OG, RL, XQ, GV, ?

SA

Explanation:

OG, RL, XQ, GV, ?

The logic followed here is :

O → (+3) → R → (+6) → X → (+6) → G → (+9) → S

G → (+5) → L → (+5) → Q → (+5) → V → (+5) → A

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