In the excerpt from Winston Churchill's speech "Their Finest Hour," he most clearly uses the audience appeal of D. Ethos. Ethos refers to an appeal to ethics, character, or credibility. In this case, Churchill emphasizes that the professional advisers of the three Services are united in advising to carry on the war, indicating a sense of trustworthiness and authority in their recommendations. By highlighting the expertise and unity of the advisers, Churchill appeals to the audience's trust in their judgment, which enhances the credibility of his message regarding the hopes of final victory in the war.