In the given scenario, Parallelogram ABDC is reflected across the x-axis to form Parallelogram A'B'D'C'. When a shape is reflected, corresponding segments maintain their parallelism.
Since segments B'D' and AB are corresponding segments after reflection (with B' coming from B and D' coming from D across the x-axis), segment AB is parallel to B'D'.
Thus, the correct answer is Segment AB.