Answer:
The air particles inside the ballon slow down due to lower thermal energy. As a result, fewer collisions take place per second and this leads to a lower pressure inside the balloon. The pressure on the outside of the balloon pushes the balloon to a smaller volume.
Step-by-step explanation:
When particles are excited by an increase in thermal energy they tend to vibrate more and collide. The charged partcles repel each other making them occupy a bigger volume. When the balloon is placed in a deep freezer, the energy is lost to the surrounding thus the particles come together occupying a smaller volume hence the size of the balloon decreases.