Select the letter-cluster that can replace the question mark (?) in the following series. AOS, CQU, ? , GUY, IWA

ESW GSW FSX DRV

ESW

Explanation: AOS, CQU, ? , GUY, IWA The logic followed here is : A → (+2) → C → (+2) → E → (+2) → G → (+2) → I O → (+2) → Q → (+2) → S → (+2) → U → (+2) → W S → (+2) → U → (+2) → W → (+2) → Y → (+2) → A   So, the letter-cluster that replaces the (?) to complete the series is "ESW"