The Letter from Birmingham Jail was an open letter written by Martin Luther King Jr. in 1963 where he talks about the nonviolent resistance to racism and emphasizes that people should work for justice and equality to come instead pleasantly wait for it. He wrote the words "Injustice anywhere is a threat to justice everywhere".
Taking this in consideration the correct answer is Separate.
Passenger transportation was required to provide equal but separated accommodations for white and colored races through separate cars or designated areas.
The other options are not correct because unequal is the opposite to the main idea of the sentence and similar is a synonym of equal.