Many cities in the Megalopolis region, such as Baltimore, Washington, and Philadelphia, are located on the Fall Line. Why were so many settlement established on or near this line?

As explained below.

Explanation:

A fall line is an upland region of the coastal plains meets and the term states that lots of rivers fall from this Piedmont to the coastal plain making rapids with waterfalls.  These falls are important as for the settlement as they block the ocean going ships from entering in any further upstream and also provided water power to run the region industries.

Answerhave flowing water either episodically or during only a portion of the year;

The correct answer is d