Explanation:

- World Ozone Day is observed annually to raise awareness about the importance of protecting the ozone layer.

- The day is also known as the International Day for the Preservation of the Ozone Layer.

- This observance commemorates the signing of the Montreal Protocol, which aims to phase out the substances that deplete the ozone layer.

- Option 1, 8 September: No significant international observance related to ozone on this date.

- Option 2, 6 September: Not related to any ozone-focused event.

- Option 3, 12 September: Not associated with World Ozone Day.

- Option 4, 16 September: This is the correct date for World Ozone Day.