What principle states that for every action, there is an equal and opposite reaction?

The principle that states that for every action, there is an equal and opposite reaction is known as Newton's Third Law. This law is fundamental in understanding how forces work between two interacting objects. When one object exerts a force on another object, the second object exerts a force of equal magnitude and in the opposite direction back onto the first object.

This concept is crucial in explaining phenomena like walking, where the feet push down on the ground, and the ground pushes back with an equal force, allowing movement. It clarifies that forces arise in pairs and reinforces the idea that interactions between objects are interconnected.

The other principles referenced, such as Newton's First Law, which pertains to inertia and the motion of objects when no net force acts upon them, and Newton's Second Law, which defines the relationship between force, mass, and acceleration, address different aspects of motion and forces. The Law of Conservation of Momentum deals with the total momentum of a closed system being constant, but it does not specifically address the action-reaction relationship that characterizes Newton's Third Law. Thus, the correct answer highlights the foundational understanding of action and reaction forces.