Final answer:
A ball rolling down a slope demonstrates potential energy being converted to kinetic energy in a Rube Goldberg machine.
Step-by-step explanation:
In a Rube Goldberg machine, potential energy is converted to kinetic energy. The correct option that demonstrates this conversion is a) A ball rolling down a slope. As the ball rolls down the slope, it loses potential energy and gains kinetic energy, which is the energy of motion.