A quadrilateral is a parallelogram if it has opposite angles that are congruent, one set of opposite sides that are both congruent and parallel, and congruent diagonals.
A quadrilateral is a parallelogram if it satisfies certain conditions. The statements that prove a quadrilateral DEFG is a parallelogram are:
Quadrilateral DEFG has opposite angles that are congruent.
Quadrilateral DEFG has one set of opposite sides that are both congruent and parallel.
Quadrilateral DEFG has diagonals that are congruent.