Answer:
A balloon powered car is moves because the balloon pushes out air. Whenever something exerts a force on something else (in other words, the balloon is exerting force on the air), then the object being pushed always exerts an equal and opposite reaction force on the object exerting force on it (in other words, the air exerts force on the balloon). Because the balloon is attached to the car, when the balloon is pushed by the air, the car is pushed.
The law that for every force there is an equal and opposite reaction force is Newton's Third Law of Motion.