The correct answer is D, as the grandfather clauses inflicted harm on African Americans with respect to mostly denial of the right to vote.
In 1866, African American slaves were emancipated. When the right to vote was granted to all citizens, it was subject to restrictions depending on each state. In seven southern states, economic and cultural measures reduced the right to vote: it could be to had a minimum area of land, or to know how to read and write the Constitution. As a derogatory measure, the right to vote was recognized for persons who did not satisfy these conditions, but who had, or whose father or grandfather already had the right to vote on 1 January 1866, that is to say prior to the American Civil War. This measure created a distortion between old and new voters, with African American as primarily targets. It was declared unconstitutional by the Supreme Court in 1915.