Were the attempts by southern states to restrict African-American voting legal under the Fifteenth Amendment?

The correct answer for the question that is being presented above is this one: "No, the attempts by southern states to restrict African-American voting is not legal." The Fifteenth Amendment states that "the right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude."