The correct answer is mT.
By definition, an air mass is a great body of air which contains a characteristic type of temperature and a given amount of moisture. In this case,mT stands for Maritime Tropical.
mT has its roots in the Gulf of Mexico and in tropical eastern Pacific as well. This air mass is totally unstable,moist and warm,too. This is the reason why it causes much of the precipitation in the central and eastern U.S.