A scientist placed an air-filled ballon into a freezer and observes that the ballon shrinks. which best explains why this happens?

D. The air particles inside the balloon move slower and push less against the sides of the balloon.

The frozen balloon shrank because the average kinetic energy of the air molecules in a balloon decreases when the temperature decreases. This makes the molecules move more slowly and have less frequent and weaker collisions with the inside wall of the balloon, which causes the balloon to shrink a little. This can be called contraction.