An air mass is unstable when:

An air mass is considered unstable when parcels of air may rise or descend spontaneously. This instability occurs because warm air, being less dense, tends to rise, and cooler air descends due to its higher density. When the surrounding environmental conditions allow for this vertical movement without significant resistance, it indicates instability within the air mass.

This can happen under specific conditions, such as when the surface temperature is significantly warmer than the air at higher altitudes, leading to the development of thermals. These thermals can cause air parcels to rise vigorously, contributing to the formation of cumulus clouds and potentially thunderstorms, as unstable conditions promote upward motion.

The other options refer to factors related to the atmosphere but do not directly define instability in air masses. For example, temperature increasing with height suggests stable conditions rather than unstable ones. Marked variations in pressure over a horizontal area relate more to weather systems and fronts than to the direct instability of an air mass. Finally, while changes in temperature and humidity can influence weather patterns, they do not inherently reflect the state of instability in terms of vertical air movement.