Explanation:

- "Spill the beans" is an idiom often used in informal contexts.

- It generally means to disclose information that was supposed to be kept confidential.

- Option 1: Reveal a plan

- Revealing a plan might include details about future actions, but "spill the beans" does not specifically imply this context.

- Option 2: Reveal a crime

- This is not the usual meaning associated with "spill the beans," although revealing a secret could sometimes involve a crime.

- Option 3: Reveal a secret

- This is the most accurate meaning. The idiom is commonly understood as telling a secret.

- Option 4: Reveal a mystery

- While revealing a mystery could involve exposing secrets, the idiom is not typically used in this way.