The by-product of photosynthesis is

Analyze the question: It asks for the by-product of photosynthesis. A by-product is a substance produced alongside the main product, but it is not the main goal of the process.

Explain the concept: Photosynthesis is the process by which green plants make food using light energy:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of sunlight and chlorophyll)

• Reactants (raw materials): carbon dioxide and water
• Main product: glucose (C₆H₁₂O₆), the food the plant makes
• By-product: oxygen (O₂), released into the air through the stomata

Step-by-step solution:

1. Identify the raw materials: CO₂ (A) and H₂O (D). These are used up, so they cannot be products.
2. Identify the main product: glucose, C₆H₁₂O₆ (C). This is the food the plant is making, so it is the main product, not the by-product.
3. The remaining substance is oxygen. It comes from the splitting of water (photolysis) during the light-dependent stage, and it is released as a waste gas.

Answer: Option B, O₂.

Tip: Remember it as "Glucose is the Goal, Oxygen is the Offcut." In photosynthesis, glucose is the main product and oxygen is the by-product. In respiration it is the reverse: CO₂ and water are released.