Afigure is a parallelogram if, and only if, it is a quadrilateral and has exactly two pairs of parallel sides.
(a "quadrilateral" is a four sided figure )
a] a figure is a parallelogram, therefore the figure is a rectangle
b] a quadrilateral has exactly two pairs of parallel sides therefore it is a parallelogram
c] a figure has two pairs of parallel sides therefore it is a parallelogram
d] a parallelogram is not a quadrilateral unless it has two pairs of parallel sides