The correct answer is the legality of racially segregated train cars.
Home Plessy was a man who broke the Louisiana state law regarding separate railroad cars for black and white passengers. Even though Plessy was considered "7/8ths white" by law, he was still not allowed to sit in the white car according to the Louisiana law. He challenged this law and his lawyer argued that this was a clear violation of the 14th amendment rights guaranteed to all citizens. However, the Supreme Court ruled that "separate but equal" was not a violation of the aforementioned amendment.