Final answer:
The reaction force to the normal force of the floor on the box is an equal and opposite normal force that the box applies to the floor, according to Newton's third law.
Step-by-step explanation:
According to Newton's third law of motion, every action has an equal and opposite reaction. This means that if the floor exerts a normal force on a box, the box also exerts an equal and opposite force on the floor. This reaction force is the force that the box applies on the floor. Since the box is resting on the floor and the force is perpendicular to the surface, this reaction force is also a normal force.
In summary, when focusing on the interaction between the box and the floor, the floor applies a normal force on the box, and by Newton's third law, the box applies an equal and opposite normal force on the floor.