Which of the following best describes Newton's Third Law?

Newton's Third Law states that for every action, there is an equal and opposite reaction. This principle highlights the reciprocal nature of forces between two objects. When one object exerts a force on a second object, the second object exerts a force of equal magnitude but in the opposite direction back on the first object. This law is essential for understanding interactions in various physical phenomena, such as the propulsion of rockets, where expelling gas downward results in an upward thrust, or the way a swimmer pushes against the water to move forward.

The other statements reflect different aspects of physics but do not accurately describe Newton's Third Law. One statement pertains to inertia, another concerns the relationship between force and acceleration, and the last one addresses mass's constancy. However, these concepts are distinct and not part of the foundation of Newton's Third Law. Thus, the correct choice emphasizes the fundamental relationship of action and reaction between two interacting bodies.