Part A: The inference that is most clearly supported by details in the passage is:
B. His confidence grew through his experience with politics in South Africa.
Part B: The excerpt from the passage that best supports the answer to Part A is:
B. Once prone to stage fright, Gandhi now showed himself to be an energetic leader. (paragraph 3)
This excerpt indicates a transformation in Gandhi's confidence, suggesting that his experiences contributed to this growth as a leader.