Final answer:
The passage that best supports the theme of not changing who you are is from the short story "Flowers for Algernon."
Step-by-step explanation:
The passage that best supports the theme that it isn't a good idea to try to change who you are is: "Artificially increased intelligence deteriorates at a rate of time directly proportional to the quantity of the increase." This passage is from the short story "Flowers for Algernon." It suggests that altering one's intelligence artificially can have negative consequences and that trying to change who you are may lead to deterioration and unhappiness.