Final answer:
The most likely person to be used in an expert testimonial ad for mouthwash is a dentist, as their professional endorsement in oral health matters carries credibility and consumer trust. Therefore correct option is B
Step-by-step explanation:
An expert testimonial ad for a brand of mouthwash would be most likely to use a dentist to endorse the product. Dentists are professionals in the field of oral health and their endorsement would carry credibility and trustworthiness. When it comes to health-related products, consumers tend to put their trust in the recommendations made by healthcare professionals, rather than a child, a popular cartoon character, or a famous sportsperson.
Therefore, a dentist would be the most effective choice for an expert testimonial ad in promoting a mouthwash brand.