Which one of the following is the largest ecosystem of the world ?

The correct answer is A, the Ocean. The marine ecosystem covers about 71% of Earth's surface and holds roughly 97% of the planet's water, so by any measure of area or volume it is the largest ecosystem. The term 'ecosystem' was coined by A.G. Tansley in 1935 for a community of organisms interacting with its physical environment, and the ocean is the biggest such system.

Why the others fail:

• Grassland (B): A terrestrial biome covering a large share of land, but far smaller than the oceans, and it is spread across several continents rather than forming one continuous system.
• Forest (C): Forests cover about 31% of the land area according to FAO's Global Forest Resources Assessment 2020. This is large on land but still a fraction of the total marine area.
• Mountains (D): These are a landform, not a single ecosystem. Mountain regions contain many ecosystems at different altitudes, so they cannot be ranked as one unit.

For Mains, this links to climate change and the blue economy. Oceans absorb a large share of excess atmospheric CO2 and heat, which makes marine ecosystem health central to climate policy and fisheries management.

Trap: Aspirants often pick Forest because the Amazon or tropical forests dominate discussions on biodiversity. Biodiversity richness and ecosystem area are different questions.