Final answer:
Word of mouth is considered the most persuasive and valuable form of marketing, supported by research in social psychology.
Step-by-step explanation:
Word of mouth is considered the most persuasive and valuable form of marketing. Research in social psychology has shown that the credibility and expertise of the speaker, as well as the trustworthiness and attractiveness of the speaker, influence the persuasiveness of a message. Famous actors and athletes are often used in advertisements to leverage their credibility and attractiveness to persuade consumers. Face-to-face communication has also been shown to be more effective in the long run.