An air mass is stable when:

An air mass is considered stable when the vertical motion of parcels of air is suppressed by the atmosphere. In a stable atmosphere, when air parcels that are pushed upwards or downwards tend to return to their original position rather than continuing to rise or sink, this indicates that the atmosphere is resisting vertical motion.

This stability is often characterized by a temperature profile that does not favor the upward movement of air. For instance, if the surrounding air is cooler than the rising air parcel, the parcel will continue to rise, ultimately indicating instability. In contrast, if the surrounding air is warmer, the air parcel will be less buoyant and will not continue to rise, thus demonstrating stability.

The other options relate to aspects of atmospheric conditions, but they do not directly define stability. For example, while constant pressure might seem like it suggests stability, it does not account for temperature gradients and vertical motion. A rapid drop in temperature with height can suggest instability, as it could lead to strong upward movements of air. Lastly, a lapse rate of 1°C per 100m is a measure that requires context to evaluate stability; it does not directly indicate stability on its own. Hence, the suppression of vertical motion is the defining characteristic of a stable air mass.