When can two bodies exert force on one another?
The bodies may or may not be in contact.
The bodies should be placed far away.
The two bodies should not be in contact.
The two bodies should always be in contact.
It is not always necessary that two bodies can exert force on one another only when they are in contact. They can be separated by any distance.
Friction can act between two bodies even when they are not in contact.
The contact force exerted by a body A on another body B is equal to the normal force between the bodies. We conclude that(a) the surface must be frictionless(b) the force of friction between the bodies is zero(c) the magnitude of normal force equal that of friction(d) the bodies may be rough but they don't slip on each other.