D. establishing separate schools for whites and blacks with equal facilities and materials
Plessy v. Ferguson decided that segregation was no unconstitutional and did not imply inequality. As long as the same materials were provided segregation could occur.
Plessy v. Ferguson was decided in 1898 and stood as the law and practice until the 1954 decision of Brown v. Board of Education. Segregation was deemed legal and expected in most states in the south. Even when there was clearly inequality between the level of the materials one would still argue it was equal. For example the white school has new books, the black school has books that are 10 years old would seemingly violate the law but politicians would argue they are equal because they both have books.