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

YZT, WXR, UVP, ?

STN

Explanation:

YZT, WXR, UVP, ?

The logic followed here is :

Y → (-2) → W → (-2) → U → (-2) → S 

Z → (-2) → X → (-2) → V → (-2) → T 

T → (-2) → R → (-2) → P → (-2) → N

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