Final answer:
The gently sloping terrain extending from Newfoundland to Alabama is known as the Coastal Plains, which is an option (C) in the question.
Step-by-step explanation:
The gently sloping Coastal plains extend some 1,600 miles from Newfoundland to Alabama. The correct answer is (C) Coastal plains. The Atlantic Coastal Plain consists of sediment accumulated since the rifting of Pangaea when the continent's eastern edge became inactive tectonically. This region, characterized by its flatness and composition of mature sediment and marine fossils, differs significantly from the rugged mountain ranges such as the Rocky Mountains that formed from orogenic activity.