Which geographic region covers the majority of the northeastern United States?

Appalachian Highlands

Explanation:

A mountain system of eastern North America extending about 2,574 km (1,600 mi) southwest from southeast Canada to central Alabama. Most of the range consists of a long band of valleys and ridges bordered on the coastal side by the Blue Ridge Mountains and the Piedmont, and on the inland side by the Allegheny Mountains, Cumberland Mountains, and the Appalachian Plateau, a dissected upland region that stretches to the interior plains. The northern Appalachians include the Green Mountains, White Mountains, and Taconic Mountains of New England.