This would be force. Acceleration is increasing the speed in an object and velocity is how fast an object is going. Also, inertia basically says that an object will stay at rest or in motion unless an outside force acts on it. So, for example, a ball will stay in the air unless gravity acts on it and pulls it down. By definition, force is any action, unopposed (or by itself without any other forces that would do the opposite) will change the motion of an object, so this definitely makes the most sense for the question. Hope this helps!
