B. It determined a person's status under the black codes.
The one-drop rule was a social and legal principle in the United States in the 20th century that stated that any person with even one ancestor of sub-Saharan-African ancestry ("one drop" of black blood) is considered black or Negro. The rule was used to determine a person's status under the black codes, which were a series of laws enacted by southern states after the Civil War to restrict the rights of newly freed African Americans.