What principle did Plessy vs. Ferguson establish?

The case of Plessy v. Ferguson established the principle of "separate but equal," which was interpreted to mean that racial segregation was permissible under U.S. law as long as the separate facilities provided for each race were equal in quality. This Supreme Court decision, rendered in 1896, upheld state laws that enforced racial segregation in public facilities, stating that segregation did not in itself constitute unlawful discrimination. This ruling effectively legitimized the Jim Crow laws in the southern United States and set a precedent that was not overturned until the mid-20th century civil rights movement, particularly with the Brown v. Board of Education decision in 1954.

The other options reference concepts that reflect subsequent legal and social changes rather than the ruling itself. For instance, the notion that separate but equal is unconstitutional arose later as societal views evolved and legal challenges were mounted against segregation. Similarly, mandatory integration of schools and equal facilities for all races were goals that emerged from the civil rights movement, opposing the precedent established by Plessy. Hence, the correct answer accurately captures the legal principle set forth by the court in the landmark case.