The correct answer is B.
After the US Civil War, the federal goverment aimed to guarantee equality of rights for all US citizens, by including such provision in the US Constitution. This happened during the so-called Reconstruction era, through the enactment of the 13th, 14th and 15th Amendments in 1865, 1868 and 1870 respectively. These amendments abolished slavery and included the Equal Protection clause, that guaranteed equality of rights for all US citizens without discrimination in terms of race, color or previous condition of servitude.
Many Southern states disagreed with the Equal Protection clause and started to issue laws which aimed to circumvent it. These are called Jim Crow laws.
For example, for preventing black citizens from voting, newly-enacted laws set voting requirements, such as a minimum income level or a literacy test. In the end, the result of such laws, was that most black citizens could not exercise their right to vote.