According to the ruling in the Plessy v. Ferguson case, why was segregation acceptable?

Segregation was legal because African-Americans were not citizens.
Segregation was legal as long as the facilities for blacks and whites were equal.
Segregation was acceptable in private facilities only.
Segregation was acceptable on a state by state basis

According to the ruling in the Plessy v. Ferguson case, segregation was legal as long as the facilities for blacks and whites were equal. This ruling established the "separate but equal" doctrine which allowed for segregation to be upheld as long as the facilities and opportunities for different races were deemed equal.