The statement that best explains the observation is: The car moves forward, while inertia keeps the balloon in place.
This reflects the concept of inertia, which is the tendency of an object at rest to stay at rest unless acted upon by a net external force. When the car accelerates forward, the balloon, which is not firmly anchored, tends to remain in its original position due to its inertia, giving the appearance that it is moving backward relative to the car.