Answer:
Great Plains.
Step-by-step explanation:
The final settlement of United States occurred around 1840's and lasted till the 1980's. It included the regions of the Great Plains and the Rocky Mountains.
During this period, the news of gold attracted many citizens towards the region. Many immigrants from Europe moved into the west because of the availability of large lands for farming, mines, etc.
Also, the government encouraged its citizens to settle in these regions by introducing various bills and amenities like the Homestead Act of 1862.