Most of the oxygen in Earth's atmosphere comes from which of the following processes?

• A. Decomposition of organic matter
• B. Volcanic outgassing
• C. Photosynthesis by plants, algae, and cyanobacteria
• D. Respiration by animals

Answer: (C)

Verified step by step guidance

1. Understand the role of oxygen in Earth's atmosphere: Oxygen is a major component of the atmosphere and is essential for life. The question asks about the primary process responsible for producing most of the oxygen.
2. Review the process of photosynthesis: Photosynthesis is a biological process carried out by plants, algae, and cyanobacteria. During photosynthesis, these organisms use sunlight, carbon dioxide (CO₂), and water (H₂O) to produce glucose (C₆H₁₂O₆) and oxygen (O₂). The chemical equation for photosynthesis is: $6_{\mathrm{CO}}+6_{H}O\to C_6{H}_{12}{O}_6+6_O$
3. Compare the processes listed in the question: Decomposition of organic matter releases carbon dioxide and methane, not oxygen. Volcanic outgassing primarily releases gases like carbon dioxide, sulfur dioxide, and water vapor, but not significant amounts of oxygen. Respiration by animals consumes oxygen and releases carbon dioxide, so it does not contribute to oxygen production.
4. Identify the correct process: Photosynthesis is the only process among the options that produces oxygen as a byproduct. Plants, algae, and cyanobacteria are responsible for generating most of the oxygen in Earth's atmosphere through this process.
5. Conclude the explanation: The correct answer is 'Photosynthesis by plants, algae, and cyanobacteria,' as this process is the primary source of atmospheric oxygen. It is essential for maintaining the balance of oxygen and carbon dioxide in the environment.