Answer: C - The frictional force exerted by the floor on the box.
To keep a heavy box sliding across a carpeted floor at constant speed, a person must continually exert a force on the box to primarily overcome the force of friction. Friction is the resistance to motion that occurs when two surfaces are in contact. In this case, the friction is between the box and the carpeted floor. The person's force must be equal and opposite to the frictional force to maintain a constant speed, according to Newton's third law of motion. This is because when an object moves at a constant speed, the net force acting on it is zero, indicating that the applied force (exerted by the person) and the frictional force are balanced.
