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.

D. Opposite angles are congruent

With option A, we can see that it is clearly wrong, in a parallelogram, only the opposite sides are congruent, if all of the sides are congruent then the shape is more likely to be a square or a rhombus.

With option B, only the opposites angles are congruent. If all four angles are congruent, that will be more likely to be true when we consider a square or a rectangle.

With option C, it is also wrong, unless the shape is a rhombus, square or rectangle.

That leaves us with the last option which is D.