Which of these events most directly resulted in expanded protections for the civil rights of African Americans in the South?

There are 4 major notable events that resulted in expanded protections for the civil rights of American Americans in the South. These events include the passage of the passing of the Wade Davis Bill in 1864, the election of Ulysses S. Grant in 1868, the impeachment of Andrew Johnson in 1868, and the ratification of the Fifteenth Amendment in 1870.