The three major physiographic provinces that make up Megalopolis are:

a. The Piedmont, the Ozarks, the Adirondacks.
b. The Appalachians, the Central Lowland, and St. Lawrence Valley.
c. The Atlantic Coastal Plain, the Piedmont, and the Appalachians.
d. The Atlantic Coastal Plain, the Pacific Border, the Columbia Plateau

C

Explanation: The Atlantic Coastal Plain, the Piedmont, and the Appalachians.

c. The Atlantic Coastal Plain, the Piedmont, and the Appalachians.

Explanation:megalopolis is a large and densely populated city or group of towns that make up an urban complex. New York City and surrounding areas including Long Island are an example of a megalopolis.

It includes the major cities of Boston, Providence, New York City, Philadelphia, Baltimore, and Washington, D.C., along with their metropolitan areas and suburbs. It is sometimes defined to include smaller urban centers beyond this, such as Richmond and Norfolk, Virginia to the south and Portland, Maine to the north.