A parallelogram is a quadrilateral in which:
Opposite sides are parallel and of equal length.
A parallelogram is a quadrilateral in which both pairs of opposite sides are parallel; a direct consequence is that those opposite sides are also equal in length.
Examining the other options:
(A) Only one pair of opposite sides parallel describes a trapezium, not a parallelogram.
(B) All four interior angles equal to 90° describes a rectangle (a special parallelogram), not parallelograms in general.
(D) Perpendicular diagonals describe a rhombus or square, not every parallelogram.
Hence the correct answer is (C) Opposite sides are parallel and of equal length.
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Important Questions from Quadrilaterals
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