what general principle was established with the Plessey v. Ferguson Supreme Court Decision?

Question

a states could not prevent African Americans from voting
b states could be punished with decreased congressional representation if they did not allow African Americans to vote
c states could require separate facilities for African Americans as long the facilities were equal
d states could not use poll taxes and literacy tests to establish voting eligibility

Answer 1

c states could require separate facilities for African Americans as long the facilities were equal