Final answer:
The sentence that contains an idiom is 'His mouth was writing checks his body couldn't cash.'
Step-by-step explanation:
The sentence that most clearly contains an idiom is:
His mouth was writing checks his body couldn't cash.
This sentence is an example of the idiom 'writing checks his body couldn't cash,' which means that someone has made promises or commitments they cannot fulfill. In this sentence, it is used figuratively to convey that the person is speaking without being able to support their words with actions or abilities.
Learn more about Idioms in English