The amount of carbon dioxide in the atmosphere is regulated by which…

The correct answer is Option (3) → Oceanic reservoir of Carbon

Option (1) Plants — Incorrect. Plants take up CO₂ in photosynthesis, but NCERT names the ocean as the regulator.

Option (2) Animals — Incorrect. Animals only add CO₂ to the air by respiration; they hold no large store of carbon. NCERT names the ocean, with 71 per cent of global carbon dissolved in it, as the regulator.

Option (3) Oceanic reservoir of Carbon — Correct. About 71 per cent of carbon is found dissolved in the oceans. This oceanic reservoir regulates the amount of CO₂ in the atmosphere.

Option (4) Burning of fossil fuels — Incorrect. Burning fossil fuels disturbs the balance by adding CO₂; it does not regulate it.

Note: This question is from the topic “Carbon Cycle” in Chapter 14, Ecosystem, of the pre-rationalised NCERT book. This portion is not in the present NCERT book, where the chapter is now Chapter 12, Ecosystem. The extract below is from the pre-rationalised edition.

NCERT Extract (Pre-rationalised edition, Chapter 14, Ecosystem):
Ecosystem – Carbon Cycle – “… When you study the composition of living organisms, carbon constitutes 49 per cent of dry weight of organisms and is next only to water. If we look at the total quantity of global carbon, we find that 71 per cent carbon is found dissolved in oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere (Figure 14.6).”