Explanation:

- The original phrase "slight openly" means the mother's mouth was "slightly open."

- Option 1: "an openly slight"

- Incorrect as it suggests something is "slight" in an open way, which doesn't describe a mouth position well.

- Option 2: "No improvement required"

- Incorrect. The original phrase is not grammatically correct for the intended meaning.

- Option 3: "open slight"

- Incorrect. This phrase is awkward and does not logically describe the way a mouth can be positioned.

- Option 4: "slightly open"

- This is the best option. It accurately and grammatically describes the position of the mouth.

Your selected answer 4: slightly open is .