The correct answer is: Exactly when it hits the ground
The correct answer is "At the very top of its bounce." At the top of its bounce, the tennis ball momentarily stops, and hence its velocity is zero. Kinetic energy is directly related to an object's velocity, so when the ball reaches its maximum height and stops momentarily, its kinetic energy is zero. To understand this, you can think about the relationship between potential energy and kinetic energy. When the ball is at the very top of its bounce, it has maximum potential energy (due to its height), but no kinetic energy (as its velocity is zero). As it starts descending, potential energy converts into kinetic energy until it once again reaches its maximum kinetic energy just before hitting the ground.
