I believe it is A because according to the theorem it states the other pair of opposite sides are parallel.
"Here's the game plan. Assume that ¯BC ¯AD and ¯BC ~= ¯AD. By definition, a parallelogram is a quadrilateral with both pairs of opposite sides parallel. You already know that one pair of opposite sides is parallel. You need to show that the other pair of opposite sides is parallel. In other words, you need to show that ¯AB ¯CD."
