An air mass usually forms in an area of. a. low pressure because its stable
b. low pressure because its unstable
c. high pressure because its unstable
d. high pressure because its stable

I think it’s b hopefully it is

D. High pressure because it’s stable

Explanation:

Air mass is volume of air which has stable temperature, humidity and pressure horizontally. Over time, each air mass acquires properties of the region by residing over same part of a surface

Areas of low pressure and high pressure occur where there is warm air and cold air rises up and cold air takes its place. Warm air has low density and pressure therefore, sinks to the bottom. This is a stable condition. The movement of air mass is responsible for maintenance of temperature conditions on Earth.