Part A: President Kennedy's main purpose for writing and delivering this speech is b. to persuade the nation to support the movement toward racial justice.
Part B: The sentence from the speech that best supports the answer to Part A 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 call to action and support for a transformative movement toward racial justice, aligning with the persuasive intent of the speech.