What is the defining characteristic of action and reaction forces?

The defining characteristic of action and reaction forces is that they occur simultaneously and are equal in magnitude, as described by Newton's Third Law of Motion. This law states that for every action, there is an equal and opposite reaction. This means that whenever one object exerts a force on a second object (the action), the second object exerts a force back on the first object that is equal in strength but opposite in direction (the reaction).

This principle is crucial for understanding interactions between objects in physics. For example, when you push against a wall, the wall pushes back with an equal force, even though it may not move. This simultaneous nature and equality in magnitude of the forces highlight the inherent balance in physical interactions, making option B the correct choice.

The other options do not accurately capture this fundamental principle. Forces are not limited to stationary objects, as they can also act on moving objects. They are not dependent solely on external factors, since the action-reaction pair is intrinsic to the objects involved. Lastly, while action and reaction forces might lead to motion, they do not guarantee that motion will be constant, as other factors like friction and resistance also play important roles.