Answer:
The appeal used was logos.
Explanation:
The logos is an appeal to logic, which makes the public of a speech be stimulated to believe in the information related to the speech because there is a logic that makes this information true.
In the case of the phrase "Nine out of ten dentists prefer Spiffy toothpaste," we can see the use of logos, as the author states to the audience that if most dentists, who are professionals, believe that Spiffy toothpaste is the best option, it is logical that the public trust and buy this toothpaste.
