Answer: the Central United States
Step-by-step explanation:
Air mass regions in the summer are usually around the middle latitudes of the Earth which means that they are closer to the Equator. This includes subtropical regions of the world including the subtropical Eastern Pacific and Western Atlantic Oceans.
Northern Mexico is also quite close to the middle latitudes so the most logical answer from the options is Central U.S. because it is the furthest from the equator and the middle latitudes of Earth.