Which of the following best describes Newton's First Law of Motion?

Newton's First Law of Motion, often referred to as the law of inertia, states that an object will remain at rest or continue in uniform motion in a straight line unless acted upon by a net external force. This law defines the behavior of objects in both states: at rest and in motion.

The first part of the law—"an object at rest stays at rest"—indicates that a stationary object will not start moving unless a force causes it to do so. The second part—"an object in motion tends to stay in motion"—means that an object moving at a constant velocity will continue to do so unless a force alters its state of motion. Both aspects highlight the principle of inertia, which is the tendency of objects to resist changes in their state of motion.

Therefore, the most comprehensive and accurate description of Newton's First Law of Motion combines both of these principles, leading to the understanding that an object at rest will remain at rest and an object in motion will continue moving in the absence of a net external force. This is why the choice that includes both components, encapsulating the essence of the law, is the best answer.