The persuasive tactic used here is Bandwagon
Explanation:
In the Bandwagon technique of persuasion widely used in advertisement, the insistence is that the audience should agree to the message of the advertisement because the majority agrees with it.
The method uses the power of majority to show what most people think should be right.
Here, the statement is that four dentists out of five recommend the gum being advertised hence the audience should buy it replying on them.
