Air masses are defined as:

• A. Large body of air with generally uniform temperature and humidity.
• B. Has distinct boundaries and may extend hundreds or thousands of km horizontally and sometimes as high as the top of the troposphere.
• C. Large volumes of air are unaffected by surrounding regions.
• D. Forms when the atmosphere has contact with a large uniform area of land or sea long enough to acquire its uniform temperature properties.

Answer: (D)

Air masses are huge parcels of air that take on the temperature and humidity of the surface they sit over. They extend over hundreds to thousands of kilometers horizontally and can reach high into the troposphere, forming when air remains in contact with a large, relatively uniform area of land or sea long enough to acquire those surface properties. That formation detail is the key: the air mass reflects the characteristics of the surface beneath it, becoming uniform in temperature and humidity across its extent. The other descriptions fall short because they either skip how the properties are acquired, imply the mass is unaffected by its surroundings, or emphasize a clear boundary that isn’t the defining feature of an air mass.