Final answer:
The key argument in the Supreme Court's verdict in the 1896 Plessy v. Ferguson case was that segregation was constitutional as long as the separate facilities were equal, establishing the concept of "separate but equal."
Step-by-step explanation:
The key argument in the Supreme Court's verdict in the 1896 Plessy v. Ferguson case was that the Louisiana law requiring railroad cars to be segregated was not a violation of the principle of equal protection under the Fourteenth Amendment. The court ruled that segregation was constitutional as long as the separate facilities were equal, establishing the concept of "separate but equal." This decision set a precedent for legally enforced racial segregation.