Select the letter-cluster that can replace the question mark (?) in the following series. DAG, FCI, ? , JGM, LIO

GDJ IFL HKE HEK

HEK

Explanation: DAG, FCI, ? , JGM, LIO The logic followed here is : D → (+2) → F → (+2) → H → (+2) → J → (+2) → L A → (+2) → C → (+2) → E → (+2) → G → (+2) → I G → (+2) → I → (+2) → K → (+2) → M → (+2) → O   So, the letter-cluster that replaces the (?) to complete the series is "HEK"