Concept:Friction is the force that resists motion between two surfaces in contact.
Explanation:Frictional force acts when two surfaces slide or try to slide over each other.
It always acts in the opposite direction to the motion, slowing down or stopping the moving object.
The amount of friction depends on how rough or smooth the contacting surfaces are.
Magnetic force is a non-contact force that acts between magnets or moving charges.
Gravitational force is a non-contact attractive force that pulls objects together, such as Earth pulling an apple downward.
Electrostatic force acts between stationary electric charges and can attract or repel them.
These three forces do not generally oppose all forms of mechanical motion.
Only frictional force directly resists and opposes motion.
Answer:B. frictional force.