The answer is A because there needs to be a complete sentence before and after it.
Medger Evers was assassinated because of the racism that existed during his time. He was an African American civil rights activist, and many people thought that what he stood up for was wrong. His life was always in danger and constant threat due to the Klu Klux Klan and other people who discriminated against him and his beliefs.
Neither is the indefinite adjective because it gives nonspecific information about the noun (one).
