Prove that quadrilateral ABCD is a parallelogram. How do you know that the figure is a parallelogram?

To prove that quadrilateral ABCD is a parallelogram, we can rely on specific properties of parallelograms. One valid way to prove this is by demonstrating that opposite sides are congruent.

If you establish that:

- Opposite sides of the figure are congruent (i.e., \(AB \cong CD\) and \(AD \cong BC\))

Then you can conclude that quadrilateral ABCD is a parallelogram based on the property that states if both pairs of opposite sides of a quadrilateral are congruent, the quadrilateral is a parallelogram.

Therefore, the correct response is:

**Opposite sides of the figure are congruent.**