I believe the correct answer is: B. It gets the audience to start thinking about whether the Declaration of Independence was beneficial for the slaves.
In this excerpt from speech “The Hypocrisy of American Slavery” delivered by Frederick Douglass, former slave and abolitionist leader, on July 4, 1852, the effect achieved by using a series of questions is to make audience think about whether the Declaration of Independence was beneficial for the slaves.
The types of questions used in this speech are called a rhetorical question, which represent the figure of speech in the form of a question that is asked to make a point rather than to elicit an answer. The proof that these questions make a point, rather than licit the answer, is the fact that Douglass says: “that an affirmative answer could be truthfully returned to these questions, then would my task be light, and my burden easy and delightful”, meaning that he does not ask of the an answer, but to give thought about whether the Declaration of Independence was beneficial for the slaves.