Which of the following letter-clusters will replace the question mark (?) in the given series?

PL, RO, TS, VX, XD, ?

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"