Final answer:
The region located in the northeastern part of the U.S. with 10 letters is New England. It is part of the Northeastern Core and the American megalopolis that spans from Washington, D.C., to Boston.
Step-by-step explanation:
The region located in the northeastern part of the U.S. that is comprised of 10 letters is New England. Included within the broader Northeastern Core, New England is known for its historical significance as an early center of colonial settlement and the birthplace of the American Industrial Revolution. The region encompasses states like Connecticut, Rhode Island, and Massachusetts and is characterized by its diverse physical environments, ranging from coastal plains to the northern Appalachians. New England is also part of the megalopolis that extends from Washington, D.C., to Boston, linking major cities and economic centers along the Northeastern seaboard. Therefore, the correct answer from the options provided is B. New England.