All the options provided—phytoplankton, soil and rocks, and the ocean—are actually carbon reservoirs. However, if you're looking for the least conventional or smallest scale in terms of carbon storage, the response might depend on context or specific criteria.
To clarify:
- Phytoplankton: These are living organisms that absorb carbon dioxide during photosynthesis, acting as a carbon sink.
- Soil and rocks: These can store carbon in the form of organic matter and mineral deposits over long timescales.
- The ocean: The ocean itself absorbs a significant amount of carbon dioxide from the atmosphere and stores it in various forms.
Since all of these are carbon reservoirs, if you need to pick one that might not fit as straightforwardly compared to the others or varies in context, you could argue for phytoplankton; however, it is still a carbon reservoir.
If there was a specific format or was looking for exclusions, please specify! Otherwise, all listed options are generally recognized as carbon reservoirs.