The answer would be:
C. An unbalanced force has acted on the vehicle.
The presence of an unbalanced force will accelerate an object, the second law of motion dictates this (Although not explicitly). Lets knock out the rest of the choices.
If a balance force acted on the vehicle, then the vehicle would be at rest. The mass of the vehicle did not change (Unless it falls apart, which I doubt). The direction of the vehicle does not change and it will only do so if another force and a stronger one at that will counteract the current net force acting on the vehicle.
Hope you got your answer here, although you did not ask for an explanation, maybe this will help you figure some of the other questions you have on your own.
