Final answer:
The corollary to Newton's first law is C) An object in motion stays in motion, reflecting the law of inertia which states that a body will maintain its state of rest or uniform motion unless acted upon by an external force.
Step-by-step explanation:
The corollary to Newton's first law, also known as the law of inertia, is C) An object in motion stays in motion. This statement encapsulates the concept that without the influence of a net external force, the motion of an object will not change. This law indicates that a body at rest will continue to remain at rest and a body in motion will continue to move at a constant velocity in a straight line unless acted upon by an external force. This fundamental principle helps to understand why objects behave as they do when in motion or at rest.