Boyle's law deals with?

Boyle's law is a fundamental principle in chemistry that describes the behavior of gases. It states that at constant temperature, the pressure of a gas is inversely proportional to its volume. This means that if the volume of the gas decreases, the pressure increases, as long as the temperature remains constant. This relationship is crucial for understanding how gases expand and contract under varying pressures, which is why option C is the correct choice.

The other options do not accurately reflect the scope of Boyle's law. Solids and liquids are governed by different physical laws, and while temperature does play a role in gas behavior, the law itself specifically addresses volume and pressure without including liquids. Lastly, the processes of fission and fusion relate to nuclear reactions, which are beyond the context of gas laws. Thus, focusing on the expansion and contraction of gases captures the essence of Boyle's law accurately.