To prove that quadrilateral ABCD is a parallelogram, we can use one of the properties that characterize parallelograms. One of the criteria for a quadrilateral to be classified as a parallelogram is that the opposite sides are congruent.
If we show that opposite sides of quadrilateral ABCD are congruent, we can conclude that ABCD is a parallelogram.
So, the response that supports our proof is:
Opposite sides of the figure are congruent.
This means if \( AB = CD \) and \( BC = AD \), then by definition, quadrilateral ABCD is a parallelogram.