Final answer:
Frederick Douglass was a key African American figure who aided the Union by recruiting soldiers and promoting the abolitionist movement, with his influential speeches encouraging enlistment and his advocacy for an inclusive America.
Step-by-step explanation:
Among the options given, the African American civilian who notably aided the Union cause by recruiting African American soldiers and supporting the abolitionist movement is Frederick Douglass. Born into slavery, Douglass escaped and became a powerful speaker, writer, and leader who utilized his literacy to argue for the rights and humanity of black people. His famous speech in Rochester implored free Blacks in the North to join the fight in the Civil War after the decision was made to recruit African American soldiers. Furthermore, his stance on integrating people from diverse backgrounds into a 'composite nation' shows his dedication to an inclusive America.
Harriet Tubman is also an exceptional figure who made significant contributions by leading enslaved people to freedom via the Underground Railroad. However, during the Civil War, Frederick Douglass's engagement with the Union cause particularly as a recruiter and orator stands out and directly relates to the question.