Final answer:
The sentence that contains an idiom is: Catherine and Gus have fallen head over heels for one another.
Step-by-step explanation:
The sentence that contains an idiom is: Catherine and Gus have fallen head over heels for one another. In this sentence, the idiom 'head over heels' is used to mean falling deeply and completely in love with someone. Idioms are phrases or expressions whose meaning cannot be understood by the individual words alone. They often have a figurative or metaphorical meaning that is different from their literal meaning.
Learn more about Idioms in English language