Let's begin with recalling one of the Conditions of a Parallelograms.
If one pair of opposite sides of a quadrilateral
is both parallel and congruent,
then this quadrilateral is a parallelogram.
We are given that in a quadrilateral ABCD sides AB and DC are parallel. Therefore, to prove that this quadrilateral is a parallelogram, these sides also need to be congruent. This corresponds with answer B.