- When a heavy load is pulled along a horizontal surface, several forces typically act upon it. - (a) The applied force in the horizontal direction: This force is necessary to initiate or sustain the pulling motion. (True) - (b) The force of gravity acting on the load vertically downwards: Gravity acts on any object with mass. (True) - (c) The force of friction acting in the horizontal direction opposite the motion: Although the surface is described as "smooth", this usually implies low friction, not necessarily zero friction in a real-world scenario or when considering all possible forces. If there is motion or a tendency for motion, friction will oppose it unless the surface is perfectly frictionless. The phrase "may be" suggests considering forces that could potentially be present. (Could be true if friction is present) - (d) Reaction force due to the horizontal surface: The surface exerts a normal force perpendicular to the surface, supporting the load against gravity. (True)