Final answer:
A soccer ball rolling up a hill best represents a system in which potential energy is increasing. Potential energy is related to the position of an object, and as the ball goes uphill, it gains height and, consequently, gravitational potential energy.
Step-by-step explanation:
The scenario that best describes a system where potential energy (PE) is increasing is c. A soccer ball rolling up a hill. As the soccer ball moves upward against the force of gravity, its height above the reference point increases, thus increasing its gravitational potential energy.
In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. For example, when lifting a rock off the ground, work is done against the force of gravity to increase the rock's potential energy.
If we consider the other options: a tennis ball hit by a racket is gaining kinetic energy; a waterfall flowing off of a cliff is converting potential energy to kinetic energy; and a boy sitting in his chair has constant potential energy assuming he is at rest relative to the Earth.