Unusual Behaviour of Less Dense Balloon in Air or Water

Answer: Option A

Of course if a heavy ball is suspended from the car we all know the ball will move in opposite direction of a decelerating car due to inertia. We are all familiar to this where inertia is applied to a body which is denser than the surrounding medium (air or liquid) which is less dense. That’s why this balloon’s behaviour surprises us!

Let’s assume the water molecules are initially moving at constant speed with the tank before the deceleration.  Due to inertia, when the tank decelerates, the water molecules continue its state of motion forward. Hence the water molecules gush to the right side of the tank, displacing (pushing) the balloon to the left. Hence the balloon moves to the left!

Refer to this Youtube video by Smarter Everyday and you can see the similar experiment of helium balloon (less denser) in the air (denser) of a car.