Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.

TOR, ?, VQT, WRU, XSV

UPS

Explanation:

TOR, ?, VQT, WRU, XSV

The logic followed here is :

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

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

R → (+1) → S → (+1) → T → (+1) → U → (+1) → V

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