The correct answer is "Southern US citizens considered black people inferior and hence resisted the provision of equal rights for them with the enactment of the Jim Crow laws.
Although equality of rights for all US citizens had been already included in the US Constitution during the Reconstruction era, through the enactment of the 13th, 14th and 15th Amendments in 1865, 1868 and 1870 respectively, such equality did not happen in practice until one century later, after the preassure exercised by the Civil Rights Movement, that led to the enactment of the Civil Rights Act in 1964.
In the meantime, Southern states have enacted the so-called Jim Crow laws that aimed to prevent black people from accesing the rights that the Constitution had granted to them. For example, those laws imposed requirements to be able to access voting rights, for example, passing literacy tests or having a minimum income level. The result was that mostly black people was excluded from voting. Southern states also implemented segregation in public facilities under "the separate but equal lemma".