Final answer:
Thyestes betrayed Atreus by committing adultery with his wife and using the act to usurp the throne, so the answer is (c) Betrayed him.
Step-by-step explanation:
The question at hand deals with the conflict between Thyestes and Atreus, two brothers whose story is rooted in ancient Greek mythology. The correct answer to the student's question is (c) Betrayed him. In the myth, Thyestes is notorious for committing adultery with Atreus's wife, Aërope, and using that as leverage to usurp the throne. This act of adultery and betrayal is a significant plot point, as it corrupts the royal lineage, which is described as a height of peril and a potential danger to the state. Atreus retaliates in a gruesome way by killing Thyestes's sons and serving them to him at a banquet, although this is not included in the answer options provided.