The correct answer is c) The addition of Hiram Rhodes Revels to the U.S. Senate.
Senator Charles Sumner, in 1870, was likely referring to the addition of Hiram Rhodes Revels to the U.S. Senate.
Hiram Rhodes Revels was the first African American to serve in the United States Senate.
He represented Mississippi and took his oath of office on February 25, 1870, after being elected by the Mississippi State Senate to fill the vacancy left by the state's secession from the Union.
Sumner was an advocate for civil rights and played a key role in the events surrounding the Reconstruction era, making the election of Revels a significant historical event.
Therefore, the correct answer is c) The addition of Hiram Rhodes Revels to the U.S. Senate.