During which condition would an air mass be least likely to change its characteristics?

An air mass remains least likely to change its characteristics when it is stagnant because it remains in a fixed location for an extended period of time. In this state, the conditions over which the air mass is situated have minimal influence on altering its properties. Factors such as temperature, humidity, and pressure will stabilize, allowing the air mass to maintain its original characteristics.

In contrast, when an air mass moves over land or water, fluctuating surface temperatures and humidities can exert significant impacts on its composition. Furthermore, encountering a topographic barrier typically leads to forced uplift, causing changes in weather patterns and possibly altering the air mass's characteristics. Therefore, the stagnation of an air mass allows it to retain its initial traits longer than any of those movements or disruptions would permit.