Final answer:
Without context from a specific book or story, one can infer that classmates would be upset if Milo used someone else's research, as it represents academic dishonesty and unfairness.
Step-by-step explanation:
We don't have a specific context or a book to reference for the details of Zach and Milo's situation, but based on the options provided for what Zach found out about Milo's science experiment that would upset the classmates, we can make some general inferences.
If the situation was that Milo used someone else's research (c), this would be academically dishonest and could upset the classmates because it compromises the integrity of the competition or project and is unfair to others who conducted their own research.