Answer:
Point Y.
Explanation:
Notice that plane UVZ is actually formed by points U, V, Z and Y. Which means point Y is part of this plane, because it's a vertex of it which help to form it.
It's important to know that point Y belongs to other planes, but in this case, we just have to identify a four point of plane UVY.
You may ask yourself, why the number of the plane has only three vertices. It is this way beacuse according to geometry, there are needed only three points to form a plane.