I would say B. It inspired him to do it by the looks of things so A wouldn't make sense. C doesn't either. I don't think excited would make sense in the sentence, so B would make the most sense. Hope that helps.
In this passage, the author implies that watching the Olympic swim meet motivated Marcus to try out. He indicates this change in disposition through the words "triggered interest." Whenever we are in doubt about the meaning of a word and we do not have a dictionary nearby, we can try to guess the meaning of the word or phrase by looking at the context of the sentence.
