Is ABCD parallelogram ?
a) Opposite sides i.e. (AB, CD) and (BC, AD) are equal.
b) Opposite sides i.e. (AB, CD) and (BC, AD) are parallel.
D. (corrected, arghhh !!!) If opposite sides are parallel then it is for sure a parallelogram. We don't need to check for opposite angles or length of opposite sides specifically as this is implicit if opposite sides are parallel