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

GRAM, EPYK, CNWI, ?

ALUG

Explanation:

GRAM, EPYK, CNWI, ?

The logic followed here is :

G→ (-2) → E → (-2) → C → (-2) → C → (-2) → A

R → (-2) → P → (-2) → N → (-2) → N → (-2) → L

A→ (-2) → Y → (-2) → W → (-2) → W → (-2) → U

M → (-2) → K → (-2) → I → (-2) → I → (-2) → G

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