Answer:
Air molecules vibrates less at cooler temperature
Step-by-step explanation:
The shape of the balloon is maintained by the molecules of the air which are vibrating, bumping in to each other and pushing the walls of the balloon. When the balloon is kept in the deep freezer, temperature drops down and air becomes cooler. The air molecules will now vibrate less and thus the force being applied on the walls of the balloon decreases.
Because of this the balloon shrinks.