Under which condition does Charles's Law apply?

Charles's Law specifically relates to the behavior of gases when the pressure is held constant and the temperature changes. It states that the volume of a gas is directly proportional to its temperature measured in Kelvin, assuming that the pressure is unchanging. Thus, as the temperature of a gas increases, its volume also increases, and vice versa.

This relationship is a fundamental concept in thermodynamics and provides insights into how gases expand when heated and contract when cooled, as long as the pressure does not vary. Each of the other conditions listed either alters the pressure or the temperature in a way that does not align with the foundational principle of Charles's Law. Therefore, the condition under which Charles's Law specifically applies is when the pressure remains constant while the temperature changes.