What type of friction force acts on objects that are not moving?

The correct answer is static friction. This force acts on objects that are at rest and prevents them from starting to move when an external force is applied. Static friction arises between surfaces that are in contact but not sliding past each other. It plays a crucial role in everyday occurrences like a book resting on a table or a parked car on a hill, where the force of static friction prevents these objects from moving until a sufficient external force overcomes it.

In contrast, sliding friction occurs when two surfaces slide against each other, while rolling friction applies to objects that roll over a surface, like a wheel on a road. Fluid friction refers to the resistance encountered when an object moves through a fluid, such as water or air. By understanding these types of friction, it becomes clear why static friction is the correct answer for objects that remain stationary.