Final answer:
Air masses are classified by the locations where they are formed, reflecting the characteristics of those regions.
Step-by-step explanation:
Air masses are classified by the locations where they are formed. This means the correct answer is D. by the locations where they are formed.
Air masses take on the characteristics of the areas over which they form, whether it is over land or water, and tropical or polar regions. These characteristics largely determine the type of weather they bring as they move.