The correct answer is B. The Rocky Mountains.
Step-by-step explanation
Maps show landforms in the United States, including the Great Plains, the Plateau, the Rocky Mountains, the Great Lakes, the Mississippi River, the Appalachian Mountains, among others. In both maps, the location of the great plains and their western border, the Rocky Mountains (brown on the map to the right), which divide the plateau, the great basin, and the southwest plains, coincide. According to the above, the correct answer is B. The Rocky Mountains.