Which Supreme Court decision ruled that blacks could not be U.S. citizens?

The Dred Scott v. Sanford case, decided in 1857, is significant in American history as it ruled that African Americans, whether enslaved or free, could not be considered U.S. citizens. This ruling came from the Supreme Court's interpretation of the Constitution, stating that slaves were property and could not sue in federal court. The case heightened tensions surrounding slavery and contributed to the onset of the Civil War.

In this context, Brown v. Board of Education addressed racial segregation in public schools, Plessy v. Ferguson upheld the "separate but equal" doctrine, and Marbury v. Madison established the principle of judicial review, none of which pertained directly to the citizenship status of African Americans. Thus, Dred Scott v. Sanford is the correct answer because it specifically addressed and denied citizenship to individuals of African descent.