The correct choice is volume, as pressure is inversely proportional to volume according to Boyle's Law. Boyle's Law states that, at constant temperature, the pressure of a gas decreases as its volume increases, and vice versa. This relationship occurs because as the volume of a gas expands, the molecules have more space to move around, resulting in fewer collisions with the walls of the container, thus decreasing pressure.
In contrast, while temperature, time, and mass play significant roles in gas dynamics and thermodynamics, they do not describe an inverse relationship with pressure in the same way that volume does. For instance, a rise in temperature at constant volume will actually increase the pressure, not decrease it. Therefore, understanding this fundamental relationship is crucial for grasping concepts related to gases and their behaviors in various physical conditions.