Hot air balloon rises because a) molecules become lighter when heated b) molecules move faster and farther apart when heated c) molecules are less attracted by gravity when heated d) molecules become charged and repel each other when heated

a. molecules become lighter when heated

Reason:

Here, a very simple scientific principle involved, that is hot air rises.When the air is heated, the density of air decreases, this causes the balloon to raise simply, because it is lighter than, an equal volume of cold air surrounds it. . Also, we know that the buoyancy is an upward force,  that the air exerts a force, so that the hot air balloons fly in air.