Why does the federal government play a major role in protecting Americans' civil rights?

A. To prevent state legislatures from demanding increased political independence

B. To uphold the standard of rational basis scrutiny in cases involving discrimination

C. To prevent state courts from changing their interpretations of the law over time

D. To ensure that no citizen of any state is denied equal protection under the law

First of all, the federal goverment aimed to achieve equality of rights for all US citizens, by including such provision in the US Constitution during the Reconstruction era, through the enactment of the 13th, 14th and 15th Amendments in 1865, 1868 and 1870 respectively.

Anyway, many states tried to circumvent such equality provisions by issuing the so-called Jim Crow laws and such equality did not happen in practice until one century later, after the preassure exercised by the Civil Rights Movement, with the enactment of the Civil Rights Act.

The Civil Rights Act was enacted in 1964 by the Federal goverment. It forbids discrimination in terms of race, color, religion, sex, or national origin. Moreover, these principles were translated into provisions that aimed to rule out such discrimination also in the practice: eliminating requirements in order to access voting, banning segregation in schools, public facilities and employment. etc.