A heavy ladder resting on a floor and against a vertical wall may not be in equilibrium, if A. Floor is smooth and the wall is rough B. Floor is rough and the wall is smooth C. Floor and wall both are smooth surfaces D. Floor and wall both are rough surfaces

The correct answer is: A. Floor is smooth and the wall is rough.

A heavy ladder resting on a floor and against a vertical wall may not be in equilibrium if the floor is smooth and the wall is rough. This is because the ladder will tend to slide down the wall, as the friction between the ladder and the wall is not enough to prevent it from moving.

If the floor is rough and the wall is smooth, the ladder will be more stable, as the friction between the ladder and the floor will be greater. However, the ladder may still slide if the angle between the ladder and the floor is too great.

If both the floor and the wall are smooth, the ladder will be the least stable, as there will be no friction to prevent it from sliding. The ladder will only be in equilibrium if the angle between the ladder and the floor is very small.

In conclusion, the correct answer is: A. Floor is smooth and the wall is rough.