We can first look into the properties of parallelogram,you can also take reference from the figure above:
-Opposite sides are parallel and are congruent.
-Opposite angles are congruent.
-The diagonals bisect each other.
We can then look into the answer:
A.Opposite angles are congurent.
-This is an correct statement.
B.Diagonals are congruent.
-This is incorrect,although they bisect each other,they do not have the same length.
C.The diagonals are peroendicular to each other.
-This is incorrect as well,not every parallelogram has the angle of 90° in the place where the diagonals bisect each others.
D.The area is equal to the product of two adjacent side-this is not true,to find the area,it have to be base×height.
Therefore the answer is A.
Hope it helps!