The answer to your question is Bandwagon. Bandwagon uses group mentality to convince individuals that product is worth purchasing. the idea is that if it's good for the group it's good for you. Plain folks is when speaker represents him/herself average, the person who can understand listener's concerns. Scientific evidence is when advertisers provide survey results to publicize their product. Testimonials is when show consists of a person's written or spoken statement about the product.
