Opinions serve as valuable sources of evidence for persuasion when they are expressed by whom?

Opinions expressed by experts serve as particularly valuable sources of evidence for persuasion because these individuals have specialized knowledge, training, or experience in a specific field. Their insights carry weight and credibility, as they are often backed by empirical research, years of practice, or extensive study. When an expert shares their opinion on a subject, such as medical advice, legal interpretations, or technical evaluations, it can significantly influence the beliefs and decisions of others due to their authority and recognized expertise. This makes expert opinions particularly persuasive in arguments or discussions, as they can provide a level of assurance and trustworthiness that is crucial for effective persuasion.