The correct answer is - A. Southwest.
The Southwest of the United States is one of the least densely populated areas in the country. The reasons for that are mainly the geography and climate of the area.
The Southwest is predominantly arid and semiarid. The rain is very rare, and the temperatures are very high. The soil is not suitable for agriculture.
This region also has mountains, plateaus, deserts, all of which are pretty inhospitable for human life, so understandably, the people avoided this place for settling down.