Answer:
c Appeals to Authority
Step-by-step explanation:
The sentence of the question is using the fallacy of appeal to authority. This fallacy establishes that something is valid only because an authority in the field says so.
In the sentence, the person will use Colgate simply because the dentist told him to do so. While this is not necessarily an unvalid reason, it would be best if the person found out by himself whether Colgate is the best toothpaste option for him or not.