Which of the following letter-clusters will replace the question mark (?) in the given series to make it logically complete?

PTA, QUB, RVC, SWD, ?

TXE

Explanation:

PTA, QUB, RVC, SWD, ?

The logic followed here is :

P → (+1) → Q → (+1) → R → (+1) → S → (+1) → T

T → (+1) → U → (+1) → V → (+1) → W → (+1) → X

A → (+1) → B → (+1) → C → (+1) → D → (+1) → E

So, the letter-cluster that replaces the (?) to complete the series is "TXE"