Which of the following shapes has only one pair of parallel sides?

A trapezoid is defined as a quadrilateral that has exactly one pair of parallel sides. This distinguishing feature sets it apart from other shapes. For instance, a rectangle has two pairs of parallel sides, while a square, being a specific type of rectangle, also has two pairs of parallel sides. A triangle, on the other hand, does not have any parallel sides at all. Therefore, the trapezoid is the only correct answer in this context, as it fulfills the criteria of possessing only one pair of parallel sides.