Which principle explains that a body at rest stays at rest, and a body in motion stays in motion?

The principle that explains a body at rest staying at rest, and a body in motion staying in motion, is known as inertia. This concept is derived from Newton's first law of motion, which states that an object will not change its state of motion unless acted upon by an external force. Inertia is the property of matter that describes an object's resistance to changes in its state of motion, whether that be remaining stationary or continuing to move uniformly in a straight line. This fundamental concept is crucial in understanding the dynamics of objects in motion, particularly in the field of air traffic control, where managing the movement of aircraft and ensuring their stability and predictable behavior is paramount. The other options do not directly capture this principle in the same way; for example, momentum refers to the quantity of motion an object has, but it does not define the condition of an object being at rest or in uniform motion.