To prove that quadrilateral ABCD is a parallelogram, the most definitive response is:
Opposite sides of the figure are congruent.
This is a property of parallelograms: if both pairs of opposite sides are congruent, then the quadrilateral is a parallelogram. If the figure you've referenced shows that AB is congruent to CD and AD is congruent to BC, then you can conclude that ABCD is a parallelogram based on this property.
If you have further information or properties indicated in the image about angles or other sides being parallel or congruent, those may also support the conclusion, but the strongest statement for establishing a parallelogram would be the congruence of opposite sides.