B. proving to his audience that other trusted people from history agree with him
Explanation:
Dr. Martin Luther King's reference to St. Augustine who was a Christian clergyman and an advocate of peace and fairness shows to his audience that other trusted people from history agree with him.
Dr. King greatly enhanced his credibility by referring such a person of authority by showing that they shared the same views.
This phrase is a metaphor "The earth is the cup of the sun, That filleth at morning" it is stating that the Earth is like a cup that in the morning will be filled by sunlight.
