Final answer:
The large body of air with low temperature and high relative humidity is called a continental polar (cP) air mass.
Step-by-step explanation:
If a large body of air has a low temperature and a high relative humidity, then the air mass is called continental polar (cP).
This classification is part of the air mass nomenclature system used by meteorologists to characterize large bodies of air based on their temperature and humidity characteristics originating over land (continental) or sea (maritime), and from polar or tropical regions.
The high relative humidity in this particular example suggests that the air mass may have picked up moisture over an ocean before moving over land, while the low temperature implies that it originated from a polar region.