What legal doctrine was established by Plessy v. Ferguson?

The doctrine established by Plessy v. Ferguson is known as "separate but equal." In this landmark case decided in 1896, the Supreme Court upheld the constitutionality of racial segregation laws for public facilities, asserting that such segregation did not violate the Equal Protection Clause of the Fourteenth Amendment as long as the segregated facilities were equal in quality.

The ruling essentially provided a legal justification for the establishment of separate facilities for African Americans and whites, reinforcing systemic racial discrimination in various aspects of life, including education and transportation. This doctrine remained in effect until it was eventually overturned by Brown v. Board of Education in 1954, which held that separate educational facilities are inherently unequal, thus violating the Equal Protection Clause.

The other choices, such as the Equal Protection Clause, refer to the constitutional provision itself that was argued in Plessy v. Ferguson but did not establish a doctrine; the "clear and present danger" doctrine relates to free speech in a different context, and "one person, one vote" pertains to electoral laws. Thus, "separate but equal" is the correct understanding of the legal doctrine that arose from this pivotal court case.