Which of the following was a cause of Americans moving west in the early to mid-1800s? A. Availability of new land for farming B. Vengeful Native Americans C. Payment by the Spanish to settle in western areas D. The War of 1812

The best answer is the availability of new land for farming in the West, while in the east, most of the land has already been taken. A very famous example for this was the Land Rushes - the races for the land that started in 1889 (that's later than the question asks, but it's connected to the question).