On a ladder resting on a rough ground and leaning against a smooth vertical wall, the force of friction acts A. Downwards at its upper end B. Upwards at its upper end C. Perpendicular to the wall at its upper end D. Zero at its upper end

The correct answer is: C. Perpendicular to the wall at its upper end.

The force of friction always acts in the direction opposite to the relative motion of the two surfaces in contact. In this case, the ladder is not moving relative to the wall, so the force of friction between the ladder and the wall acts perpendicular to the wall.

Option A is incorrect because the force of friction would not act downwards at the upper end of the ladder. If the force of friction acted downwards, it would cause the ladder to slide down the wall.

Option B is incorrect because the force of friction would not act upwards at the upper end of the ladder. If the force of friction acted upwards, it would cause the ladder to lift off the wall.

Option D is incorrect because the force of friction would not be zero at the upper end of the ladder. If the force of friction were zero, the ladder would be able to slide down the wall.