The large connected urban area from boston to washington, dc, is often referred to as a(n)

This area is called Megalopolis, and more specifically the Northeast Megalopolis.

This includes Boston in the North of the Megalopolis, New York in the center and Washington in the south. It also includes Baltimore and Philadelphia.

About 50 Million people live there. 

Idon't know. the question you are trying to ask doesn't make sense.

C. naturally in the environment