The correct option is (a).
Explanation
Air is a mixture of gases constituting the Earth's atmosphere. Oxygen is a critical component for biological and chemical processes, distinct in its function and abundance compared to other atmospheric gases.
Statement 1 is Correct: Oxygen is essential for cellular respiration in aerobic organisms. In this process, oxygen is utilized to break down food (glucose) to release energy required for life activities, producing carbon dioxide and water as by-products.
Statement 2 is Incorrect: The burning of fuels (combustion) is an oxidation process that consumes oxygen. It does not produce oxygen; instead, it typically releases carbon dioxide, water vapor, and energy in the form of heat and light. Oxygen is primarily produced by plants during photosynthesis.
Statement 3 is Incorrect: Oxygen is not the most abundant gas in the atmosphere. It constitutes approximately 21% of the air by volume. Nitrogen is the most abundant gas, comprising about 78% of the atmosphere.
Key Takeaway: Oxygen is vital for respiration and supports combustion, but it is the second most abundant atmospheric gas, surpassed significantly by Nitrogen.