Correct Answer:
D. To reveal a secret
This question tests your knowledge of English idioms. The idiom \"let the cat out of the bag\" means to accidentally or deliberately reveal a secret that was supposed to remain hidden.
The origin of this expression is commonly linked to old marketplaces, where dishonest sellers allegedly substituted a cat for a pig in a bag. Opening the bag exposed the deception, giving rise to the idiom. Today, it is used whenever confidential information is disclosed before the appropriate time.
- Idiom: Let the cat out of the bag.
- Meaning: Reveal a secret.
- Example: He let the cat out of the bag about the surprise party.
Therefore, the correct answer is: \"To reveal a secret\".