A body will begin to move down an inclined plane if the angle of inclination of the plane is ______ the angle of friction

nature of surface only

Coefficient of friction depends upon nature of surface only, it is independent of area of contact.

equal to

The force required to move the body up the plane will be minimum if it makes an angle with the inclined plane equal to the angle of friction.

upward at its upper end

A ladder is resting on a smooth ground and leaning against a rough vertical wall. The force of friction will act upward at its upper end.

zero at its upper end

A ladder is resting on a rough ground and leaning against a smooth vertical wall. The force of friction will act zero at its upper end.