If this question has the same list of options as to the following, the answer would be "Agamemnon's selfish behavior angers Achilles, who finally decides that he can no longer fight for the king." It's letter D.
Options are:
A. Agamemnon insults Achilles for refusing to fight with his comrades, which spurs Achilles to prove himself to Agamemnon when he joins the war.
B. Agamemnon kills Achilles's best friend, Patroclus, so Achilles seeks vengeance on Agamemnon in the war.
C. Agamemnon thinks that Achilles's idea to build a wooden horse is foolish, but Achilles goes against the king and the Greeks topple Troy as a result.
D. Agamemnon's selfish behavior angers Achilles, who finally decides that he can no longer fight for the king.