Final answer:
The reaction force to the gravitational force exerted on an apple at rest on a table is the equal and opposite gravitational force that the apple exerts on the Earth, as described by Newton's third law of motion. Because Earth is so massive compared to the apple, the reaction force does not result in a noticeable movement of Earth.
Step-by-step explanation:
If we call the gravitational force exerted on an apple at rest on a table the action force, then the reaction force, according to Newton's third law of motion, is the force that the apple exerts on the Earth. Newton's third law states that for every action, there is an equal and opposite reaction. Thus, while Earth exerts a gravitational force on the apple (action), the apple also exerts an equal and opposite gravitational force on Earth (reaction).
Since the gravitational force is equal to the mass of the apple times the acceleration due to gravity (approximately 9.8 m/s²), the reaction force is of the same magnitude but directed upwards towards the apple. This reaction force is often not noticed because the mass of Earth is so immense in comparison to the mass of the apple that the acceleration Earth experiences due to this force is negligible.
The concept of the normal force also becomes relevant here. The normal force is the support force exerted by a surface, in this case, the table on the apple, which balances the apple's weight and prevents it from falling. It is equal in magnitude and opposite in direction to the gravitational force (weight) exerted by the apple on the table.