Final answer:
The force the ball exerts on the paddle is equal in magnitude but opposite in direction to the force exerted by the paddle, which is -50 N, following Newton's third law of motion.
Step-by-step explanation:
When Ana hits the ping pong ball with the paddle and the paddle exerts a force of 50 N on the ball, according to Newton's third law of motion, for every action, there is an equal and opposite reaction. Therefore, the ball will exert an equal force on the paddle in the opposite direction. The correct answer to the question "What force does the ball exert on the paddle?" is B. -50 N. The negative sign indicates the direction of the force is opposite to that exerted by the paddle. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. Therefore, the force exerted by the ball on the paddle is equal in magnitude but opposite in direction to the force exerted by the paddle on the ball.