How can potential energy be transferred into kinetic energy?

"When an object falls, its gravitational potential energy is changed to kinetic energy. You can use this relationship to calculate the speed of the object’s descent.  

For example, say that you lift a 40-kilogram cannonball onto a shelf 3.0 meters from the floor, and the ball rolls and slips off, headed toward your toes. If you know the potential energy involved, you can figure out how fast the ball will be going when it reaches the tips of your shoes."

option number c is the right answer.