The correct answer is B) the availability of farmland.
During the 19th century, the US was constantly expanding westward. Due to the idea of Manifest Destiny, the federal government strived to add as much territory to the US in order to connect the two sides of the continent. This resulted in the US gaining a significant amount of territory.
This increase in territory resulted in the federal government creating policies that encouraged American citizens to move west. One example would be the Homestead Act, which gave 150 acres of land to any American willing to move west.