Part A: The correct answer is B. to persuade the nation to support the movement toward racial justice.
Part B: The sentence from the speech that best supports this answer is D. A great change is at hand, and our task, our obligation, is to make that revolution, that change, peaceful and constructive for all. (paragraph 4). This sentence emphasizes the need for a collective effort towards progress in racial justice, aligning with the purpose of persuasion.