Which logical fallacy is used in the text?
The two authors of this study claim that social media can be healthy for adolescents. However, neither author attended a top-ranked medical school, so you can't trust their research.

 - straw man: a misrepresentation of an opponent's position that makes it easier to argue against
 - circular reasoning: an argument that supports a claim with the claim itself
 - ad hominem: an attack against the person making the argument, rather than the argument itself
The text argues that the two authors' research on adolescents and social media can't be trusted because neither author attended a top-ranked medical school. This is a personal attack on the authors' qualifications that isn't relevant to whether their research is valid. This illustrates a type of logical fallacy known as ad hominem.