Final answer:
The reaction force does not cancel the action force because they are exerted on different bodies, highlighting a principle of Newton's third law of motion. The correct option is 'D. they are on different bodies'.
Step-by-step explanation:
The question pertains to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. These reaction forces do not cancel the action forces because they act upon different bodies. For instance, when a swimmer pushes off from the side of a pool, the swimmer exerts a force on the pool wall, and an equal but opposite force is exerted by the wall on the swimmer.
Although these forces are equal in magnitude and opposite in direction, they do not cancel each other out as they are acting on separate systems. The correct option that explains why the action and reaction forces don't cancel each other is D. they are on different bodies. This concept is fundamental in understanding the interactions and movements in a variety of scenarios, from aquatic locomotion to aerospace propulsion.