The antagonist is usually the main villain. So If I had to chose one, I'd choose answer choice d.) would be correct, the villain is the main source of conflict and problem in the story. But technically choice c.) is also correct, they react negatively to events in the story.
Hope this helps! :)
Please note that sometimes the antagonist can be the main character, but usually it is the protagonist or the hero. The antagonist can also change to support the protagonist, but again, this is not typical.
The definition of antagonist is "<span>a person who actively opposes or is hostile to someone or something" so t</span>he antagonist causes the conflict in the story would be the most likely answer.
