Final answer:
The Appalachian Mountains divided the Civil War into two main theaters: east to the Atlantic and west to the Mississippi.
Step-by-step explanation:
The correct answer is B) Appalachian Mountains.
The Appalachian Mountains divided the Civil War into two main theaters: east to the Atlantic and west to the Mississippi. These ancient mountains, formed during multiple collisions with small island chains during the Paleozoic, span most of the Southeastern United States. The Appalachian Mountains can be further divided into four distinctive physiographic provinces based on their topography and geology.