Answer:
f. 500N
Explanation:
The force exerted by the wagon on the horse is 500N directed in the opposite path .
This is premised on the Newton's third law of motion which states that "action and reaction forces are equal and opposite in direction".
So, as the action force of 500N is used by the horse to drag the wagon, the wagon also exerts an oppositely directed force of 500N on the horse.
This way, the acceleration can continue.