The gas in air used by plants for photosynthesis is

Step 1: Analyze the question The question asks which gas in air plants take in and use for photosynthesis.

Step 2: The concept Photosynthesis is the process by which green plants make food (glucose) using light energy absorbed by chlorophyll.

Word equation: Carbon (IV) oxide + Water → (sunlight, chlorophyll) → Glucose + Oxygen

Chemical equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Step 3: Check each option

• A. Argon: a noble gas that is inert, so plants do not use it.
• B. Carbon (IV) oxide: this is a raw material of photosynthesis. Plants absorb it from the air through their stomata. ✔
• C. Nitrogen: it makes up about 78% of air, but plants cannot use it directly. Bacteria must first convert it into nitrates (nitrogen fixation).
• D. Oxygen: this is a product of photosynthesis, not what the plant uses for it. Plants use oxygen in respiration.
• E. Water vapour: plants get the water for photosynthesis from the soil through their roots, not from the air.

Step 4: Answer The gas in air used by plants for photosynthesis is Carbon (IV) oxide, which is option B.

Tip: Remember that photosynthesis takes in CO₂ and gives out O₂, while respiration does the opposite. In exams, if the question says "used by plants for photosynthesis," think CO₂. If it says "released," think O₂.