Friction opposes motion. What force acts in the opposite direction when an object moves?

Frictional force is the key concept here, as it directly opposes the motion of an object. When an object moves, friction acts in the opposite direction to the applied force or the direction of motion. This opposing force is a result of the interactions between the surfaces in contact, which can be influenced by factors like the nature of those surfaces and the normal force acting between them.

While gravitational force acts downward towards the center of the Earth, and applied force can be in various directions depending on how a force is exerted on the object, they do not specifically counteract the motion. Spring force pertains to the restoring force in a spring, which also does not oppose the motion of an object directly. Therefore, recognizing that friction is the force that resists motion and acts directly opposite to it highlights why it is the correct answer in this context.