Which of these sentences is always true for a parallelogram? a. all sides are congruent.
b. all angles are congruent.
c. the diagonals are congruent.
d. opposite angles are congruent.

The correct option is D.

Step-by-step explanation:

A paralellogram is a flat shape with 4 straight sides and opposite sides are parallel.

There are six important properties of a parallelogram.

1) Opposite sides are congruent.

2) Opposite angels are congruent.

3) Consecutive angles are supplementary.

4) If one angle is right, then all angles are right.

5) The diagonals of a parallelogram bisect each other