Final answer:
A quadrilateral is a rectangle if, and only if, its diagonals are congruent and perpendicular and they bisect each other. A quadrilateral is a rhombus if, and only if, its diagonals are perpendicular.
Step-by-step explanation:
a. A quadrilateral is a rectangle if, and only if, its diagonals are congruent and perpendicular and they bisect each other.
b. A quadrilateral is a rhombus if, and only if, its diagonals are perpendicular.