What Was the Basis for the Supreme Court's Decision in Plessy

What was the basis for the Supreme Court's decision in Plessy v. Ferguson (1896) , which upheld the constitutionality of a state law requiring segregated railroad facilities?

A) Former slaves are not entitled to full citizenship rights because they did not immigrate to the United States willingly.
B) The Constitution requires that states treat people equally, not that states must place different people together.
C) Railroad transportation involves interstate commerce, which is regulated by Congress; no provision in federal law prohibits segregation.
D) Each state has the right to interpret the Constitution as it sees fit, as long as the interpretation is "reasonable and without malice."