Answer: d) to provide a contrast to his group’s nonviolent efforts.
The author states that various black nationalist groups are springing up across the nation. He also states that these represent the force of "bitterness and hatred" and that they come "perilously close to advocating violence." It is obvious from the passage that the author disagrees with these groups and believed them to be violent and bitter. It is likely that he wants to provide a contrast to his own group's nonviolent efforts.
