The answer is:
A - Slavery
The "great act" referred to by Senator Sumner in this excerpt is "The addition of Hiram Rhodes Revels to the U.S. Senate."
What Senator Charles Sumter said in 1870 was the following: "All men are created equal, says the great Declaration and now a great act attests this verity. Today we make the Declaration a reality. ... The Declaration was only half established by Independence. The greatest duty remained behind. In assuring the equal rights of all we complete the work."
Hiram Revels became the first African American to enter the Senate of the United States Congress when the Republicans supported him on February 25, 1870.
