The best explanation for the observation that the balloon appears to move backward when the car starts moving forward is:
The car moves forward, while inertia keeps the balloon in place.
This statement accurately describes the concept of inertia, which is the tendency of an object to remain at rest or to continue moving in a straight line at a constant speed unless acted upon by an external force. In this case, the car accelerates forward, but the balloon, due to inertia, tends to remain in its original position, creating the perception that it moves backward relative to the car.