Question

What is the dominant mountain range on the East Coast?

Answer 1

That would be the Appalachian Mountains.

Explanation:

The Appalachian Mountains run all the way from Alabama into eastern Canada. Including the associated plateaus they touch twenty USA states and five Canadian provinces, plus the French islands of St. Pierre and Miquelon off the shores of Newfoundland. The Appalachian Mountains are often considered relatively low, but Mount Mitchell in North Carolina rises more than 2 km above sea level.

See https://en.wikipedia.org/wiki/Appalachian_Mountains.