Final answer:
The air mass type not found in the United States is (e) Maritime tropical, as the climate in the U.S. includes polar, tropical, continental polar, and maritime polar air masses.
Step-by-step explanation:
The type of air mass not found in the United States is (e) Maritime tropical. The United States experiences a variety of air masses, including polar, tropical, continental polar (cP), and maritime polar (mP). However, the Maritime tropical (mT) air masses, typically found in tropical and subtropical regions, do not originate within the United States but can affect the country's weather, particularly in the southeastern regions. Air masses are large bodies of air that are defined by the temperature and moisture content they carry, forming over land or ocean areas and being classified by their source region's characteristics.