Explanation:

- Carbon dioxide is the primary cause of ocean acidification. When CO2 is absorbed by seawater, it reacts with water to form carbonic acid, which lowers the pH of the ocean.

- Carbon monoxide doesn't contribute significantly to ocean acidification; it's more of a pollutant impacting air quality.

- Hydrogen sulfide can be toxic and cause localized issues but is not significant on a global scale for acidification.

- Chlorofluorocarbons (CFCs) primarily affect the ozone layer and don't have a direct role in acidification processes.

Answer: Option 1 - carbon dioxide