Answer: The answer is AB = CD.
Step-by-step explanation: We are given a quadrilateral ABCD with AB ║ CD.
We know that a quadrilateral is said to be a parallelogram if any one of the following conditions is satisfied:
(i) If both pairs of opposite angles of a quadrilateral are equal, then it is a parallelogram.
(ii) If one pair of opposite sides of a quadrilateral is both equal and parallel, then it is a parallelogram.
(iii) If the diagonals of a quadrilateral bisect each other, then it is a parallelogram.
We are given that AB is parallel to CD, so by the condition (ii), if we get the additional information that AB = CD, then ABCD will be a parallelogram.
Thus, the answer is AB = CD.
