Final answer:
A rectangle has congruent diagonals because they bisect each other and are equal in length.
Step-by-step explanation:
A rectangle has congruent diagonals because the diagonals of a rectangle bisect each other and are equal in length. To prove this, let's consider a rectangle ABCD. The diagonals AC and BD intersect at point O. We can show that triangle AOB is congruent to triangle COD by using the Side-Angle-Side (SAS) congruence. Since these triangles are congruent, their corresponding sides are equal, which means that the diagonals AC and BD are congruent.