Which shape is defined as a quadrilateral with exactly one pair of parallel sides?

A trapezoid is defined as a quadrilateral that has exactly one pair of parallel sides. This characteristic sets trapezoids apart from other quadrilaterals, such as rectangles, parallelograms, and squares, which have two pairs of parallel sides.

In a trapezoid, the non-parallel sides can be of different lengths, and the angles can vary. This flexibility in the shape of the trapezoid allows for variations such as isosceles trapezoids, where the non-parallel sides are of equal length, but still, the key feature remains that only one pair of sides is parallel.

This distinct definition is what confirms that the trapezoid is the correct answer to the question, while other options like rectangles, parallelograms, and squares do not meet this specific criterion. In those shapes, the presence of two pairs of parallel sides is a defining characteristic, making them different from trapezoids.