Answer:
According to Newton's third law of motion, the reaction force is equal to the action force.
Explanation:
Newton's third law of motion states that for every action there is an equal and opposite reaction. This means that if one object exerts a force on a second object, the second object exerts an equal force in the opposite direction. When Plate X pushes on Plate Y with the action force (Fx), Plate Y must push back with an equal force (Fy).
Therefore, the best explanation is that according to Newton's third law of motion, the reaction force is equal to the action force.