The author repeated the idiom about butterflies in Luisa's stomach three times throughout the story to show how Luisa's feelings of nervousness were decreasing as the day went on.
Idioms are groups of words that relate to a meaning which is not directly deducible. In other words, idioms express certain meanings that may or may not be literal but can be figurative.
Having butterflies in one's stomach means a person is nervous about a certain thing. And as such, the mentioning of butterflies in Luisa's stomach thrice throughout the story shows how her nervousness reduces as the day goes on. Thus, the correct answer is option C.
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