Which statement is a true statement a. a trapezoid is a regular polygon b. not all rectangles are parallelograms c. not all parallelograms are rectangles d. a trapezoid is a parallelogram

C. Not all parallelograms are rectangles.

A is incorrect because trapezoids are not regular polygons (they do not have all equal sides and angles), B is incorrect because all rectangles are always parallelograms, and D is incorrect because a trapezoid is not a parallelogram (it does not have two pairs of parallel sides).