Answer: B. to highlight the hypocrisy of celebrating freedom when slavery is still practiced in America.
Hypophora is a figure of speech in which a writer states a question, but immediately responds to it. Most of the time, the question is asked in the first paragraph, and answered in the following ones. In this case, Douglass presents a question ("What to the slave is the Fourth of July?") and then proceeds to answer it in order to highlight the hypocrisy of celebrating freedom when slavery is still practiced in America.