Carbon is most abundant in the
carbon is most abundant in the
and
and

Carbon is most abundant in the atmosphere and hydrosphere.  

Explanation:

In the atmosphere, carbon is mostly present in the gaseous state of carbon dioxide. Plants absorb atmospheric carbon through photosynthesis and prepare food which is used by other living organisms. Respiration by plants and animals returns carbon back to atmosphere.

Next to atmosphere, carbon is abundant in the hydrosphere dissolved in water. Aquatic plants and marine organisms absorb carbon dissolved in water to prepare food. This carbon is also used to produce shells and other marine structures.

Carbon is also present in lithosphere and biosphere.

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