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

AR, CU, EX, GA, ?

ID

Explanation:

AR, CU, EX, GA, ?

The logic followed here is :

A → (+2) → C → (+2) → E → (+2) → G → (+2) → I  

R → (+3) → U → (+3) → X → (+3) → A → (+3) → D  

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