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 at rest will remain at rest, and an object in motion will continue in a straight line at constant speed unless acted upon by an unbalanced external force. This principle highlights the concept of inertia, which is the resistance of any physical object to change in its state of motion or rest.

The first part of the correct choice emphasizes that if no external force acts on a stationary object, it will remain stationary. This is crucial in understanding the behavior of objects in equilibrium. The second part indicates that if an object is in motion, it will not change its motion unless influenced by an unbalanced force.

The other options do not fully encapsulate the entire concept of inertia as presented by Newton's first law. While they might touch on aspects of motion or forces, they fail to accurately summarize the law's essence. The simplicity of stating that an object remains at rest or in uniform motion unless acted upon by an external force clearly conveys the principle of inertia as defined by Newton.