The sentence that contains an idiom would be:
We didn’t tell Marcella about Kim’s surprise party because we were sure she’d let the cat out of the bat and tell Kim about it.
An idiom is a form of figurative language in which exaggerated or non literal word choices are used to emphasize a point. They are not meant to be taken seriously.
With that in mind, we can eliminate A and D because A does not have any figurative language, and D is simply incorrect because there is a correct option.
We also can eliminate B because an idiom and a simile are two different subjects, although they both are under the category of figurative language. Similes are sentences or phrases that connect two unrelated instances by using the words “like” or “as.” In answer B, the word “as” is used connect the courtroom’s silence to that of a library.
Answer C is correct because the phrase “let the cat out of the bag” is a discrete and implicit way of saying that something would spoil a secret. This is the case in this option.
Answer C contains an idiom.
