Explanation:

- Overt: Something that is open, obvious, or not hidden.

- Option 1, Unambiguous: Clearly defined or explicit. Not an antonym to overt, as both imply clarity.

- Option 2, Concealed: Hidden or kept out of sight. This is the opposite of overt.

- Option 3, Manifest: To display or show something clearly. Similar to overt, not an antonym.

- Option 4, Inexplicable: Impossible to explain or understand. Not directly related to visibility or openness.

- Correct Answer: Option 2, Concealed