The correct answer is: "Voting Rights Act".
The Reconstruction Amendments (13th, 14th and 15th) abolished slavery, granted equal protection and rights to US citizens and prohibited the goverment at the federal and state levels from impeding any citizen to vote because of his race, color or previous condition of servitude (respectively).
But in practice, many Southern states tried to circumvent the mentioned provisions by enacting the so-called Jim Crow laws. These tried to indirectly prevent African Americans from voting by setting requirements in order to have access to vote, such as, a minimum income level or passing a literacy test. The majority of people who did not meet the predefined criteria were black citizens.
The US Supreme Court kept on overturning these laws, but as soon one of them was prohibited, a new one was ready. This mechanism was not stopped until the enactment of the Voting Rights Act in 1975, which finally brought equality of rights in voting for all US citizens, in practice (because those had already been included in the Constitution amendments enacted a century before).
