If the two ropes each exert a force of 5 N upward, the total upward force is 10 N. Since the forces are balanced and the box is stationary, the box must exert an equal and opposite force downward. According to Newton's third law, the force exerted by the box itself is 10 N downward.
So, the correct response is:
The box exerts 10 N of force downward.