The best representation of potential energy being converted to kinetic energy is b) a child holds a toy above her head and then lets it fall to the ground.
Explanation: When the child holds the toy above her head, it possesses gravitational potential energy due to its height above the ground. Once she lets it go, that potential energy is converted into kinetic energy as the toy accelerates towards the ground. This is a clear and straightforward demonstration of the transformation between potential and kinetic energy.