The vertices of a parallelogram are A(x1, y1), B(x2, y2), C(x3, y3), and D(x4, y4). Which of the following must be true if parallelogram ABCD is proven to be a rectangle?

2 answers

I don't see any "following". If you tried to copy and paste please be advised that this site does not support copy and paste.
well, a rectangle has 4 right angles
it also has two pairs of parallel congruent sides

so pick the choice which specifies that.
I mean, you do know what a rectangle is, right?