Final answer:
The letter 'f' in the free-body diagram should represent the 'frictional force', which is equal in magnitude and opposite in direction to the pulling force, ensuring the box moves at a constant speed.
Step-by-step explanation:
In the context of the question describing a box being pulled with a constant speed, the free-body diagram should show forces that are balanced since there is no acceleration. The box has a weight of 120 N and is being pulled by a force of 24 N.
Since the box is moving at a constant speed, the force of friction and the pulling force must be equal and opposite to ensure there is no net force acting on the box (Newton's first law of motion applies as the velocity is constant). Thus, the letter f in the diagram should be replaced with frictional force, which would be equal in magnitude to the pulling force of 24 N but directed oppositely.