Answer:
A) Keeping slaves makes slaveholders inhuman.
Step-by-step explanation:
Frederick Douglass was a writer, orator (and others), famous for his Narrative of the Life of Frederick Douglass, an American Slave, which focuses on all of the slaveholders doings, and how slaves were treated.
The main idea of the book would be that slaveholders are made inhuman for keeping slaves.
Note: we must not forget that D is true, but that it is not the main focus of the book. It is also true that Fredrick Douglass himself helped recruiting coloured men for the Civil War.
Hope it helped,
BiologiaMagister