Answer: A. Positive ions
An ocean is a reservoir of carbon. All of the following bring carbon into the ocean except positive ion:
Photosynthesis: Photosynthesis is conducted by aquatic plants in the ocean to prepare their food which is a carbon product.
Decaying organisms: The aquatic plants and animals are buried beneath the ocean bed and they are decayed and decomposed by the activity of microbial organisms. The organic matter produced after this process is rich source of carbon.
The gases in the atmosphere: The gases in the atmosphere consist of carbon dioxide and carbon monoxide released in the atmosphere by burning of fossil fuels, greenhouse gas emission and by other process. These gases are dissolved into ocean water making it available for photosynthesis conducted by aquatic plants. Hence, carbon from the atmosphere enters in ocean through these atmospheric gases.