What type of force acts opposite the motion of a body?

Kinetic friction is the force that acts opposite to the direction of motion of a body that is already in motion. This type of friction occurs when two surfaces are sliding against each other. For instance, when a block slides across a table, kinetic friction opposes the motion of the block, ultimately slowing it down.

The key factor that distinguishes kinetic friction from other types, such as static friction, is that kinetic friction is in effect when the object is moving, while static friction acts to prevent motion before it occurs. Dynamic friction is often used interchangeably with kinetic friction, but in some contexts, dynamic friction may also refer to friction during continuous movement or to more complex scenarios involving movement through fluids. Rolling friction, on the other hand, pertains specifically to the resistance encountered by rolling objects, such as wheels on a surface, and is generally less than the friction experienced by sliding.

Thus, the correct answer highlights the specific type of friction that counteracts the motion of an already moving object.