Final answer:
The unexpected arrival of the antagonist is another important event caused by the main problem.
Step-by-step explanation:
The event that belongs in the summary because it is another important event caused by the main problem is: d) The unexpected arrival of the antagonist. In a summary, it's essential to focus on events directly related to the main problem or conflict. The unexpected arrival of the antagonist typically introduces a significant development or complication that drives the plot forward and is directly tied to the central issue or conflict in the story.