c. The magnitude of the force of the car on the fly is equal to the magnitude of the force of the fly on the car.
This statement is based on Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. When the fly smacks into the windshield of the car, it exerts a force on the windshield (action), and in turn, the windshield exerts an equal and opposite force on the fly (reaction). The magnitudes of these forces are equal.