Final answer:
The Justice Department blocked South Carolina's new law because it was felt that the law might prevent minorities from voting, reflecting concerns about new voter ID laws' potential to discriminate. The best option to fill the empty box labeled Cause is A. The U.S. Justice Department felt that the law might keep minorities from voting.
Step-by-step explanation:
The Cause for the Justice Department blocking South Carolina's new law is that The U.S. Justice Department felt that the law might keep minorities from voting. This is based on concerns that new voter identification laws, as seen in several states after the Supreme Court repealed aspects of the 1965 Voting Rights Act, could disproportionately affect minorities and the poor. Challenges to these laws suggest they might impose undue burdens on voters, akin to past efforts to limit minority voting such as poll taxes during the Jim Crow era.