Answer:
A. Speaker's credentials
Explanation:
The passage applies ethos, a persuasive speech technique that references the credibility of the speaker to make the audience confident in the information they are about to hear because an authority in the field is the speaker.
By talking about the profession of the speaker as well as his background in research, the audience is thus told that the person who is about to speak is knowledgeable in his field, therefore, they can trust that whatever e is about to say is factual and accurate.
