A force that opposes motion between two surfaces is known as:

Friction is a force that resists the relative motion or tendency of such motion of two surfaces in contact. It acts parallel to the surfaces and opposes the direction of motion or applied force. This force arises due to the interactions at the microscopic level between the surfaces of the materials, which may include factors such as surface roughness and the presence of any contaminants or lubrication.

Inertia, on the other hand, is a property of matter that describes an object's resistance to changes in its state of motion, meaning it does not actively oppose motion itself but quantifies an object's tendency to remain at rest or in uniform motion unless acted upon by an external force.

Gravity is a force of attraction between masses, drawing objects towards one another, typically towards the center of the Earth, and is not specifically related to opposing motion at the contact surface.

Acceleration is the rate of change of velocity of an object and does not function to oppose motion; instead, it describes how quickly an object speeds up, slows down, or changes direction.

Therefore, friction is the correct answer because it is the specific force that directly opposes the motion between two surfaces in contact.