The correct answer is D. action force.
In the context of rocket propulsion, when the rocket expels gas downward (the action), it creates an equal and opposite force that pushes the rocket upward (the reaction). This is based on Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. Thus, the act of forcing air (or exhaust gases) out of the bottom of the rocket acts as the action force, resulting in the lift-off of the rocket.