as it air is cooled, a balloon will get smaller.
to better understand this, we should recall the properties of gases
when we have pressure, volume and temperature, they are all proportional to each except pressure and volume.
so, if air is heated, meaning temperature is increased, that means volume increases. (get's bigger-- not the answer)
if balloon rises, as it goes up, the pressure decreases, if pressure decreases, volume increases
when the air is cooled, meaning temperature is decreased, that means volume decreases as well.