The low pressure area on the map is: A.
The air pressure represents the density and the weight of the air. The more dense the air mass is, the more it weighs, thus the pressure is higher, and the lower the density of the air mass, the less it weighs, thus the pressure is lower.
The standard air pressure is 1013.25 mbar. Anything above it is considered as high air pressure, and anything bellow it is considered as low air pressure. On this map, the spot that has air pressure lower then 1013.25 mbar is the one marked with A, while all the others have high air pressures as they have higher values then 1013.25 mbar.
