The correct answer is A.
Brown V. Board of Education was a landmark decision enacted by the US Supreme Court in 1954, that abolished segregation in public schools and understood that the 'separate but equal' principle that had governed such procedures was violating the Equal Protection Clause and therefore, unconstitutional. This clause was introduced by the 14th amendtment to the US Constitution during the Reconstruction Era, aiming to guarantee equality of rights to all US citizens.
This decision (in 195), overturned the former Plessy v. Ferguson decision from 1896, that had understood that the 'separate but equal' principle did not violate the Equal Protection clause and, therefore, it enabled segregation because it stated that Congress did not have power to ban it when public segregated facilities were comparable in quality.