How do gases behave when they undergo cooling?

When gases undergo cooling, they experience a decrease in temperature, which leads to a reduction in kinetic energy of the gas molecules. As the kinetic energy decreases, the molecules move more slowly and are less energetic, causing them to occupy less space. This results in the gas contracting or shrinking in volume. The behavior of gases during cooling aligns with Charles's Law, which states that, at constant pressure, the volume of a gas is directly proportional to its temperature in Kelvin. Therefore, as the temperature drops, the volume of the gas also decreases, illustrating the contraction behavior of gases.

In contrast, the other options do not accurately reflect the behavior of gases under cooling conditions. While gases might appear to remain unchanged at a specific temperature, significant cooling results in a contraction, not stability. Though some gases can eventually become solids at very low temperatures, this phase change occurs at exceptionally low temperatures, rather than as a direct result of standard cooling processes. Lastly, the notion that gases expand contradicts the fundamental principles of thermodynamics. Thus, contraction is the clearly defined and consistent behavior of gases when subjected to cooling.