The sentence from "The Quiz Bowl King" that shows confidence that leads to a downfall is:
C. I feel as cool as a cucumber, running through the list of possible topics in my head—pop culture, animals, psychology, astronomy, technology—and knowing I am an expert on every one.
This sentence expresses a high level of confidence in the speaker's knowledge, which could ultimately lead to a downfall if it results in overestimating their abilities or underestimating the competition.