What principle does inertia illustrate regarding objects in motion?

Inertia is a fundamental principle in physics that describes the behavior of objects in motion or at rest. It states that an object will maintain its state of motion (whether that means staying at rest or moving at a constant velocity in a straight line) unless acted upon by an external force. This can be observed in everyday scenarios: a car that is cruising along the road will continue to move forward until the driver applies the brakes (an external force), and an object sitting on a table will remain there until someone picks it up or pushes it.

In this context, the correct choice highlights the dual nature of inertia. It indicates that not only does an object in motion stay in motion unless a force acts on it, but it also emphasizes that an object at rest will remain at rest unless a force causes it to move. Therefore, both statements regarding motion and rest illustrate the principle of inertia and form a comprehensive understanding of the concept. Since option D combines both aspects of inertia, it encapsulates the principle accurately by acknowledging that both scenarios are relevant to understanding how objects behave.