Which of the following processes releases oxygen to the atmosphere?

1. Analyze the question We need the process that releases oxygen into the atmosphere. Most of the other options use oxygen up.

2. Explain the concept Oxygen is added to and removed from the atmosphere by different processes:

• Oxygen-consuming processes: respiration, combustion (burning) and decomposition (microbes respire as they break down dead matter).
• Oxygen-producing process: photosynthesis, in which green plants, algae and some bacteria make food using light.

Photosynthesis equation:

6CO₂ + 6H₂O → (light, chlorophyll) → C₆H₁₂O₆ + 6O₂

Oxygen is released as a by-product when water is split during the light-dependent stage.

3. Solution, option by option

• A. Respiration: uses O₂ and releases CO₂. ❌
• B. Combustion: burning uses O₂ and releases CO₂ and heat. ❌
• C. Decomposition: decomposers respire aerobically, so they consume O₂. ❌
• D. Photosynthesis: takes in CO₂ and releases O₂. ✅

4. Answer The correct option is D, photosynthesis.

5. Exam tip Remember the opposites: photosynthesis gives O₂, while respiration, combustion and decomposition take O₂. If a question asks about oxygen being released, think of green plants in sunlight. If it asks about CO₂ being released, think of the other three.