Which of the following gases is essential for the process of photosynthesis?

Correct answer: (b) CO2

Explanation

Carbon dioxide is essential for photosynthesis: it is the source of the carbon in the sugar the plant makes.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in light, with chlorophyll). Oxygen is a product, not a raw material.