Final answer:
At its highest point, Mario's soccer ball has maximum gravitational potential energy, due to the height it has reached against Earth's gravity. This potential energy is converted from the initial kinetic energy with which Mario kicked the ball, and will convert back to kinetic energy as the ball falls.
Step-by-step explanation:
Mario kicks his soccer ball straight up into the air, and at its highest point, the ball possesses gravitational potential energy. This form of energy is highest when the ball is at the peak of its trajectory because it has been lifted against the force of gravity. At the highest point, the ball's kinetic energy is zero because it momentarily stops before changing direction and falling back toward the ground. As the ball rises, it loses kinetic energy and gains gravitational potential energy; once it starts falling, this potential energy is converted back into kinetic energy.
An object has maximum potential energy at its highest point. The Principle of Conservation of Mechanical Energy tells us that the mechanical energy of the system, which includes kinetic energy and gravitational potential energy, remains constant if air resistance is neglected. Thus, the energy Mario initially imparted to the ball in the form of kinetic energy is converted entirely into gravitational potential energy at the top of the ball's path, and then back into kinetic energy as the ball falls.