A trapezoid is the only type of quadrilateral that has exactly one pair of parallel sides. Therefore, among the options listed, the correct answer is:
- a) trapezoid
To explain further, a trapezoid (or trapezium in some regions) is defined by having just one pair of sides that are parallel. The other two sides are not parallel, which is what distinguishes it from other quadrilaterals like parallelograms, which have two pairs of parallel sides. The other mentioned shapes—rhombus, square, rectangle, and parallelogram—all have two pairs of parallel sides and do not fit the criteria of having exactly one pair.