What does gas compressibility refer to?

Gas compressibility refers to the ability of a gas to decrease in volume when subjected to increased pressure. This phenomenon occurs because the molecules in a gas are relatively far apart and can be pushed closer together under pressure. As a result, when external pressure is applied, the distance between gas molecules reduces, leading to a decrease in the overall volume of the gas, demonstrating its compressibility.

In contrast, the other options describe different behaviors that gases exhibit but do not accurately define compressibility. For instance, maintaining volume under pressure implies that the gas is incompressible, which contradicts the inherent nature of gases. The notion of expanding without limit highlights another property of gases, known as expansibility, rather than compressibility. Remaining unchanged under temperature variations suggests a lack of response to temperature changes, which is not specific to compressibility. Thus, the ability to decrease in volume under pressure is what makes option C the correct understanding of gas compressibility.