Final answer:
A ball falling to the ground is not an example of periodic motion as it does not repeat its motion in cycles and is a one-time event.
Step-by-step explanation:
The example that is not an example of periodic motion is D. a ball falling to the ground. This is because periodic motion involves a system that repeats its motion over a regular cycle or interval of time. The other choices (A. an object orbiting the sun, B. a pendulum swinging, and C. a mass on a spring) all describe systems that exhibit repetitive oscillatory movement and thus can be classified as periodic motion. A ball simply falling to the ground under the influence of gravity does not repeat its motion; it is a one-time event unless an external force acts upon it to elevate it back to its original position.