Data
AB= 6
BC=12
CD=6
DA=12
The data determine the measure of the segments so if you draw them you can see if it is a parallelogram, as follow:
A paralellorgram is a figure where the pairs of opposite sides have the same measure.
In this case the opposite sides are:
AB and CD
BC and DA
So then, ABCD is a parallelogram because his opposite sides have the same measure