Which of the following letter-clusters will replace the question mark (?) in the given series? PL, RO, TS, VX, XD, ?

ZL ZK YK ZJ

ZK

Explanation: PL, RO, TS, VX, XD, ? The logic followed here is : P → (+2) → R → (+2) → T → (+2) → V → (+2) → X → (+2) → Z L → (+3) → O → (+4) → S → (+5) → X → (+6) → D → (+7) → K   So, the letter-cluster that replaces the (?) to complete the series is "ZK"