Which logical fallacy is used in the text?
How do I know that Karen is the most intelligent person in our geometry class? I know because she's so smart.

 - circular reasoning: an argument that supports a claim with the claim itself
 - false dichotomy: an argument that presents only two choices when more options exist
The text argues that Karen is intelligent because she's smart. However, the "evidence" is just a restatement of the claim itself. This illustrates a type of logical fallacy known as circular reasoning.