Final answer:
The magnitude of the reaction force of the rope pulling on the horse is 1200 N, which is equal to the force that the horse exerts on the rope.
Step-by-step explanation:
If a horse tied to a post pulls on the rope with a force of 1200 N, according to Newton's Third Law of Motion, there will be a reaction force equal in magnitude but opposite in direction exerted on the horse by the rope. Hence, the magnitude of the reaction force of the rope pulling on the horse is also 1200 N. This principle is similar to the problem-solving strategy used for Newton's laws of motion, as the reaction force and the force exerted by the horse are an action-reaction pair.