Part A:
The correct answer is B. to persuade the nation to support the movement toward racial justice.
Part B:
The sentence 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 need for collective action toward racial justice and the importance of making that change constructive and peaceful, aligning with Kennedy's intention to persuade the nation to support civil rights.