A property of a parallelogram is that their diagonals bisect eachother. Since in part a the diagonals bisect eachother, we know for sure that it's a parallelogram.
We also know that you can make two congruent triangles with a parallelogram. Note that if you draw a line from one accute angle to the other, we create two equal triangles by SAS congruency, so since the figure is comprised of two congruent triangles, it is a parallelogram.
