The choice that best explains how the anaphora in the speech excerpt advances the speaker’s purpose is:
It serves as a call to action that appeals to diverse groups of people.
In the passage, Obama's repeated emphasis on "all of us" highlights the collective responsibility of every individual, regardless of their background, to engage in the work of change. This use of anaphora emphasizes inclusivity and encourages a unified response to the issues at hand, aligning with his purpose of promoting social justice and equality.