Which is the most stable ecosystem?

The answer is A (Marine/ocean). Stability here means how little the ecosystem's physical conditions and community structure swing over time. The ocean is the most buffered environment on Earth. Its huge volume absorbs heat, so temperature changes slowly, and salinity, light and pressure vary far less than on land.

• Forest: Reasonably stable and biodiverse, especially tropical rainforests, but exposed to seasonal drought, fire, storms and human clearing. It is less buffered than the ocean.
• Mountain: Fragile. Steep altitudinal gradients, thin soils, low temperatures, high UV and short growing seasons make it sensitive to small changes.
• Land: Too broad to be a single ecosystem type. Terrestrial systems in general show sharp daily and seasonal extremes, which is why the option is wrong.

Core concept: ecosystem stability, covering resistance (ability to withstand disturbance) and resilience (ability to recover). Ocean ecosystems have high environmental buffering even though they face new pressures such as warming and acidification. For Mains, this links to climate change and why marine ecosystems are both vulnerable and critical to carbon and heat regulation.

Trap: Aspirants pick Forest because it is biodiverse and 'green'. Biodiversity is not the same as stability. The ocean's environmental uniformity is what the question tests.