Final answer:
Opposite angles of a parallelogram are congruent when the lines forming the parallelogram are parallel.
Step-by-step explanation:
In order to prove that opposite angles of a parallelogram are congruent, we need to show that the alternate interior angles formed by the transversal lines m and n are congruent.
Given that m || n (line m is parallel to line n) and l || k (line l is parallel to line k), we know that the alternate interior angles are congruent.
Therefore, opposite angles of a parallelogram are congruent.