Most of the world's carbon is found:

1. as CO2:
- Carbon dioxide (CO2) is a greenhouse gas that is naturally present in the Earth's atmosphere.
- It is released through natural processes such as respiration, volcanic activity, and the decay of organic matter.
- Human activities, such as the burning of fossil fuels and deforestation, have significantly increased the concentration of CO2 in the atmosphere.
- CO2 is an important component of the carbon cycle, as it is absorbed by plants during photosynthesis and released back into the atmosphere through respiration and combustion.

2. in living organisms:
- Carbon is a fundamental element of life and is found in all living organisms.
- It is a key component of organic molecules, such as carbohydrates, proteins, lipids, and nucleic acids, which are essential for the structure and functioning of cells.
- Carbon is cycled through the biosphere as organisms consume organic matter and release carbon dioxide through respiration.
- The carbon in living organisms can be found in various forms, including organic compounds, biomass, and inorganic carbonates.

3. as carbonate and bicarbonate ions dissolved in the ocean:
- The ocean acts as a carbon sink, absorbing a significant amount of carbon dioxide from the atmosphere.
- When CO2 dissolves in seawater, it reacts with water molecules to form carbonic acid (H2CO3), which then dissociates into bicarbonate ions (HCO3-) and carbonate ions (CO32-).
- These carbonate and bicarbonate ions are important components of the ocean's carbonate system, which plays a crucial role in regulating the pH and chemistry of seawater.
- Marine organisms, such as corals and shell-forming organisms, utilize carbonate ions to build their skeletons and shells, thereby sequestering carbon in the form of carbonate minerals.

4. as carbonate minerals in sedimentary rocks:
- Over long periods of time, the accumulation of organic matter and the deposition of carbonate minerals in marine environments can lead to the formation of sedimentary rocks.
- Carbonate minerals, such as limestone, dolomite, and chalk, contain significant amounts of carbon in the form of carbonate ions.
- These rocks store large quantities of carbon and play a crucial role in the long-term carbon cycle.
- Through geological processes such as weathering, erosion, and subduction, carbon can be released from sedimentary rocks back into the atmosphere as CO2 or transported to the ocean through rivers.

In conclusion, carbon exists in various forms and reservoirs on Earth, including as CO2 in the atmosphere, in living organisms, as carbonate and bicarbonate ions dissolved in the ocean, and as carbonate minerals in sedimentary rocks. These reservoirs are interconnected through the carbon cycle, which regulates the distribution and cycling of carbon in the Earth system.

3rd option