Which quadrilateral has one pair of parallel sides?

A trapezoid is defined as a quadrilateral that possesses at least one pair of parallel sides. This characteristic distinguishes trapezoids from other types of quadrilaterals, such as rectangles, squares, and rhombuses, which have more specific properties.

In a rectangle, both pairs of opposite sides are parallel, while a square, which is a special type of rectangle, shares this characteristic along with equal side lengths. A rhombus also qualifies as a type of parallelogram, meaning both pairs of its opposite sides are parallel. The defining feature of a trapezoid, however, is its unique condition of having just one pair of parallel sides, making it the correct answer in this scenario.