The Underground Railroad wasn't a real railroad, but a network of courageous people who helped slaves secretly travel from the South to the North. In 1850, President Fillmore signed the Fugitive Slave Act, which made it illegal to help a slave run away and required officials to return escaped slaves to their owners.
The answer would be B, Plains Farmers
The Mound Builders were generally known for "<span>d. building large cities," since they would build "mounds" that were used for a variety of purposes--religious, residential, etc. </span>
I believe this was influenced by the Britain's First prime minister.