Final answer:
The best supporting excerpt is when Marian Anderson felt a great wave of goodwill from her audience, indicating a positive shift in attitudes towards African Americans.
Step-by-step explanation:
The excerpt from Marian Anderson Sings that best supports the idea that Anderson witnessed a positive shift in the attitude of many Americans towards African Americans is: "had a feeling that a great wave of good will poured out from these people, almost engulfing me." This passage vividly illustrates the overwhelming support and kindness that Marian Anderson felt from the audience during her performance at the Lincoln Memorial, which symbolized a broader change in public sentiment during that era. It reflects the societal shift and greater acceptance that were part of the changing landscape of civil rights in the United States.