Answer:
The diagonal of the rectangle must be equal to the diameter of the circle.
Step-by-step explanation:
The point where the diagonals of a rectangle cross will be the center of a circumscribing circle. Obviously, the vertices of the rectangle lie on the circle, so either diagonal will be a diameter of the circle.
In order for it to be possible to inscribe a rectangle in a circle, its diagonal measure must match the diameter of the circle.
