Hi there!
[tex]\large\boxed{\text{Friction.}}[/tex]
We can go through each:
1) Inertia is NOT a force. Inertia is defined as an object's resistance to motion, or its mass.
2) Friction IS a force because it results in acceleration.
3) Acceleration is not a force, but rather a product of force.
4) Velocity is simply a quantity that can be changed as a result of a force.
