Photosynthesis fixes atmospheric carbon into organic matter for food chains

Steps:

• Identify the carbon cycle: Carbon moves between atmosphere, organisms, and Earth via processes like fixation and release.
• Define "returns carbon to a food chain": Means incorporating carbon into producers (plants) that herbivores and others consume.
• Evaluate choices: Check which process builds organic compounds from CO2 for entry into trophic levels.
• Select based on role: Photosynthesis alone converts inorganic CO2 into biomass accessible to the food chain.

Why C is correct:

• Photosynthesis uses CO2 + H2O → glucose + O2 (6CO2 + 6H2O → C6H12O6 + 6O2), fixing carbon into sugars that form the base of food chains.

Why the others are wrong:

• A. Combustion: Burns organic carbon, releasing CO2 back to atmosphere, not into food chain.
• B. Decomposition: Breaks down dead matter, releasing CO2 via microbial respiration, exiting the food chain.
• D. Respiration: Organisms release CO2 from organic compounds, returning carbon to atmosphere, not food chain.

Final answer: C