The correct answer here is that Frederick Douglass was the most famous black abolitionist before the Civil War and the most famous black male suffrage leader after it.
John Brown cannot possibly be the right answer. There are two reasons for this. The first reason is that John Brown was not African American. He was a white man who hated slavery. The second reason is that John Brown died before the war even started. He was executed for his raid on the armory at Harper's Ferry, Virginia.
Frederick Douglass was an African American who escaped from slavery and became a very famous advocate for black rights before and after the war.