Which of the following is the primary source of oxygen in the Earth's atmosphere?

• A. Respiration by animals
• B. Photosynthetic activity of plants and cyanobacteria
• C. Volcanic outgassing
• D. Decomposition of organic matter

Answer: (B)

Verified step by step guidance

1. Understand the question: The problem is asking for the primary source of oxygen in Earth's atmosphere. Oxygen is a key component of Earth's atmosphere and is essential for life.
2. Recall the biological process responsible for oxygen production: Photosynthesis is the process by which plants, algae, and cyanobacteria convert carbon dioxide and water into glucose and oxygen using sunlight. This is the primary source of atmospheric oxygen.
3. Eliminate incorrect options: Respiration by animals consumes oxygen and releases carbon dioxide, so it cannot be the source of oxygen. Volcanic outgassing releases gases like carbon dioxide and sulfur dioxide, not oxygen. Decomposition of organic matter also releases carbon dioxide and methane, not oxygen.
4. Focus on the correct option: Photosynthetic activity of plants and cyanobacteria is the correct answer because these organisms produce oxygen as a byproduct of photosynthesis.
5. Conclude: The primary source of oxygen in Earth's atmosphere is the photosynthetic activity of plants and cyanobacteria, which has been occurring for billions of years and is responsible for the oxygen-rich atmosphere we have today.