Yes. It can be assumed that Sumner had the support of the people as the legislature was popularly elected. The legislature also passed Sumner's own Objections to the Mexican War. This appears to be another sign that he was respected and had the support of the voters of Massachusetts.
Uncle Tom's Cabin; or, Life Among the Lowly is an anti-slavery novel by American author Harriet Beecher Stowe. Published in 1852, the novel had a profound effect on attitudes toward African Americans and slavery in the U.S. and is said to have "helped lay the groundwork for the Civil War".
The answer is C. Having a secondary source from a time that an event occurred can help historians recreate and study the event.