The statement that best explains the observation is:
The car moves forward while inertia keeps the balloon in place.
This is due to the principle of inertia, which states that an object at rest will stay at rest unless acted upon by a net external force. When the car accelerates forward, the balloon tends to remain in its initial position (relative to the ground) due to its inertia, making it appear as though it is moving backward relative to the car.