Which one of the following is an artificial ecosystem?

The correct answer is A) Rice field. A rice field is an agroecosystem. Humans create it by clearing land, levelling, bunding, transplanting, irrigating and adding fertilisers and pesticides. Its species composition is chosen by the farmer, and it collapses into a natural wetland or fallow land if that human input stops. That dependence is what makes it artificial.

Why the others are wrong:

• B) Forest is a natural terrestrial ecosystem. It regenerates and sustains itself through its own nutrient cycling, even when some management is involved.
• C) Grassland is natural too, dominated by grasses and maintained by rainfall, fire and grazing. Savannas and steppes are natural biomes.
• D) Lake is a natural lentic freshwater ecosystem formed by geological and hydrological processes. A man-made reservoir or pond is a different case, and that is the trap.

The core concept is the distinction between natural and artificial (managed) ecosystems. For Mains, paddy fields link to food security, water use, methane emissions from flooded fields, and sustainable intensification methods such as SRI.

Trap: aspirants treat any ecosystem touched by humans as artificial, or they pick a man-made lake or reservoir without noticing that the question asks about a category of ecosystem.