Which statement about friction is correct?

Friction is a force that acts at the interface between two surfaces in contact, and it plays a crucial role in the motion of objects. The correct statement is that friction aims to resist motion. This means that when two surfaces slide against each other or are in contact and an attempt is made to move one relative to the other, friction opposes this movement.

For instance, when you try to push a heavy box across the floor, friction between the box and the floor works against your push, making it harder to move the box. This resistance is important for various reasons; for example, it allows vehicles to grip the road surface, preventing them from sliding uncontrollably.

The other statements do not accurately describe the nature of friction. It does not always increase speed, as its primary role is to oppose motion. While friction generally acts on solid objects, it can also be present in fluids; thus, saying it only acts on solid objects is too limiting. Friction is not restricted to instances when objects are at rest; it can occur whether the objects are moving or stationary, such as when an object is sliding down a slope.