If two opposite angles of a quadrilateral are supplementary, is the quadrilateral a parallelogram?
A No, because in parallelograms, angles are
supplementary if and only if they are consecutive.
B No, because opposite angles in a parallelogram are
never congruent.
C Yes, but only when the angles of the quadrilateral are
right angles. Otherwise, it will be a trapezoid rather
than a parallelogram.
D Yes, because only squares have that property, and
squares are both parallelograms and quadrilaterals.