Question

What doctrine justified legal segregation in the South?

Answer 1

The correct answer is: "separate but equal"

The "separate but equal" doctrine governed the proliferation of segregated public schools and other facilities under the belief that, if facilities were equal in quality, such system was not violating the equality of rights provision that had been guaranteed for all US citizens by the Reconstruction Amendments to the US Constitution.   

The separate but equal principle was adopted after the decision enacted by the US Supreme Court in Plessy v. Ferguson case  in 1896.