Question

what, to the slave, is the fourth of july?
fellow - citizens, pardon me, allow me to ask, why am i called upon to speak here to - day? what have i, or those i represent, to do with your national independence? are the great principles of political freedom and of natural justice, embodied in that declaration of independence, extended to us?
diction
formal
why might douglass have decided to make his diction so formal?
to make himself seem more realistic
to help his audience visualize his ideas
to frustrate people with limited verbal skills
to take the sting out of his critical message

Explanation:

Formal diction is often used to convey seriousness and respect. By using formal diction, Douglass could present his message in a more respectful and serious tone, which might make the audience more receptive to his critical message about the lack of freedom and justice for his people. It takes the "sting" out in the sense that a more formal and respectful approach (rather than an angry or confrontational one) might make the audience less defensive and more open to listening to his points. Making himself seem realistic is not a typical reason for formal diction. Visualizing ideas is more related to descriptive language (not diction level). Frustrating people with limited verbal skills is an unkind and unlikely motive for a serious speaker like Douglass.

Answer:

To take the sting out of his critical message.