B) Use the slope formula to prove the slopes of the diagonals are opposite reciprocals.
Step-by-step explanation:
A distance formula is used to find the length of a line segment. Knowing diagonal lengths cannot tell you they are perpendicular, eliminating choices C and D.
Diagonals of a quadrilateral are never parallel, so choice A makes no sense.
The slopes of the diagonals of a quadrilateral will be opposite reciprocals if the diagonals are perpendicular. The slope formula can be used to show that is the case. Choice B is the appropriate answer.
