The correct answer is "anti-slavery."
From the letter, it can be inferred that Mr. Phillips is anti-slavery.
We are talking about the letter written by Wendel Pillips, who was a man that supported abolitionism, civil rights and founded the American Anti-slavery Society. This is a letter addressed to Frederick Douglass. He expressed the importance of Douglass's work "Narrative," as a way of making people pay attention to the issue of slavery and creating consciousness of the big problem it represented in the United States.
