The relationship between the forces exerted by the ball on the bat and by the bat on the ball is described by Newton's Third Law of Motion. This law states that for every action, there is an equal and opposite reaction.
Therefore, the correct answer is:
F_ball on bat = -F_bat on ball
This indicates that the force exerted by the ball on the bat is equal in magnitude but opposite in direction to the force exerted by the bat on the ball.