What do frictional forces always do in relation to motion?

Frictional forces are inherently characterized by their tendency to oppose the motion of an object. When an object moves across a surface, the microscopic irregularities between the object and the surface create resistance, which is the essence of friction. This resistance works against the direction of motion, thereby slowing down or stopping the moving object.

In practical terms, if you push a box across a floor, the friction between the box and the floor acts in the opposite direction to your push. This opposition requires additional force to maintain or initiate movement. Understanding this interaction is crucial for analyzing scenarios involving motion, as friction plays a key role in dynamics, such as stopping a vehicle or initiating movement on a surface.

Therefore, the correct choice highlights the fundamental nature of friction as a force that consistently acts to counteract any applied forces that aim to promote motion.