Select the letter-Cluster that can replace the question mark (?) in the following series. ACD, EGH, IKL, ?, QST

MNO MOP NPQ NOP

MOP

Explanation: ACD, EGH, IKL, ?, QST The logic followed here is : A → (+4) → E → (+4) → I → (+4) → M → (+4) → Q C → (+4) → G → (+4) → K → (+4) → O → (+4) → S D → (+4) → H → (+4) → L → (+4) → P → (+4) → T So, the letter-cluster that replaces the (?) to complete the series is "MOP"