Which of the following is a good analogy for quantization using gravitational energy? a. a ramp b. a set of stairs c. an apple falling from a tree d. a swinging pendulum

The answer for this is C. an apple falling from a tree. 

Because once, an apple fell on young Isaac Newton's head and that was when he came up with his theory of gravity. That incident was momentous in the history of science. That falling apple inspired Newton and physicists in understanding gravitational force.