The reason a hot air balloon sinks is because when air is cooled It expands, causing its density to decrease
It contracts, causing its density to decrease
it expands, causing its density to increase

It contracts, causing its density to increase

I’d say A. It expands, causing its density to decrease

But I’m not sure so I’m sorry if it’s wrong.

As the molecules heat and move faster, they are moving apart. So air, like most other substances, expands when heated and contracts when cooled. Because there is more space between the molecules, the air is less dense than the surrounding matter and the hot air floats upward.

Heating air causes it to spread out and become less dense. The warmer, less dense air rises because colder, denser air pushes it out of the way.

Idont know but good luck; )