What event challenges Oedipus's belief about his parents?

The event that challenges Oedipus's belief about his parents is indeed the claim made by a drunken man at a party. This encounter introduces Oedipus to the unsettling rumor that he is not the biological son of King Polybus and Queen Merope, his supposed parents. This revelation creates an internal conflict for Oedipus, as it raises doubts about his identity and the legitimacy of his royal status. The drunken man's claim serves as a catalyst, prompting Oedipus to seek the truth about his origins, ultimately leading him to confront the prophecies and the tragic fate that awaits him.

The other options, while significant in their own right, do not directly challenge Oedipus's understanding of his parentage. A warning from a priest or an encounter with a prophecy reflects more on the theme of fate and destiny rather than directly altering Oedipus's belief about who his parents are. A letter from a foreign ruler does not play a foundational role in his character arc regarding his lineage but may pertain to political matters instead. Therefore, the drunken man's assertion is pivotal as it sets the stage for the unraveling of Oedipus's tragic story.