In a quadrilateral, if a pair of opposite sides is equal and parallel, then it is possibly a
Rhombus
Square
Parallelogram
In a square, rhombus and parallelogram both the pairs of opposite sides are equal and parallel. And as per the question, only one pair is equal and parallel which automatically makes the other pair of opposite sides equal and parallel. Hence, the quadrilateral can be a square, rhombus or a parallelogram.