The correct response is:
B of the availability of farmland.
In the early 1800s, many people migrated westward primarily seeking opportunities for new land and agricultural prospects, as the availability of farmland was a significant motivation for expansion. The Transcontinental Railroad was not completed until the 1860s, so options C and D are not accurate reasons for early 1800s westward migration. While some people sought to escape slavery, it was not a primary motivation for most migrants at that time.