Final answer:
A rock rolling down a hill exemplifies potential energy transforming into kinetic energy, as the rock's stored energy due to its elevated position is converted to the energy of motion as it descends. So the correct option is A.
Step-by-step explanation:
Among the options presented, the example of potential energy being transformed into kinetic energy occurs when a rock sitting at the top of a hill starts rolling down the hill. This is reflected in choice A: A rock sitting at the top of a hill rolling down the hill. Initially, the rock at the top possesses potential energy due to its elevated position in Earth's gravitational field. When it begins to roll downhill, this potential energy is converted into kinetic energy, which is the energy of motion.
As the rock continues to roll down, it gains speed, and hence, more kinetic energy. This transformation is a fundamental concept in physics and can also be seen in other scenarios such as water falling over a waterfall or a roller coaster car descending from its highest point.