Final answer:
The gas expelled from the hot air balloon expanded and its volume increased as it mixed with the cooler atmospheric air, following Charles's Law.
Step-by-step explanation:
When Mark and Wanda's hot air balloon operator let some of the hot air escape to descend, the gas expanded and its volume increased. This occurs because gas molecules move more freely and take up more space when they are not confined, a concept known as Charles's Law, which states that a gas will expand when the temperature drops as long as the pressure is constant. As the gas leaves the heated environment of the balloon and mixes with the cooler atmospheric air, it expands and the volume increases, since it is no longer under the same pressure as it was inside the balloon.