The word that would best fit into the blank is (a.) scapegoat.
The word 'scapegoat' refers to a person forced to bear the blame for others. In Toni Morrison's novel, Sula appears as a scapegoat for the decline of her community due to the fact that she behaves as a nonconformist. The people of the Bottom blame Sula for all the misfortunes that affect the community; therefore, she is made the town's scapegoat, which, at the same time, they believe that will help them to become a better community.