Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.

BEC, CID, FOG, KUL, ?

RAS

Explanation:

BEC, CID, FOG, KUL, ?

The logic followed here is :

B → (+1) → C → (+2) → F → (+5) → K → (+7) → R

E → (+4) → I → (+6) → O → (+6) → U → (+6) → A

C → (+1) → D → (+3) → G → (+5) → L → (+7) → S

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