Answer: WZ
The sequence ABCD has AD as the first and last letters of that sequence, so this pattern applies to WXYZ as well to pull out WZ. Angle A pairs up with angle W because they are both the first letters of the sequences. These angles are congruent from the fact that the parallelograms are similar. The same can be said for angle D and angle Z.
note: WZ is the same as ZW. The order doesn't matter when it comes to labeling segments based on endpoints.