If a tropical rain forest is removed, it does not regenerate quickly as compared to a tropical deciduous forest. This is because

The correct answer is A. In a tropical rain forest most of the nutrients are locked up in living biomass, not in the soil. Heavy rainfall leaches nutrients out of the thin topsoil, and the soil is typically lateritic and poor in humus. When the forest is cut, the rapid nutrient cycling that sustained it breaks down, so the land cannot quickly rebuild fertility for regeneration. Tropical deciduous forests generally sit on richer soils and lose nutrients less severely.

• B is wrong: there is no general evidence that rain forest propagules have poor viability. Viability is not the reason for slow recovery.
• C is partly true but not the cause asked: many rain forest species are slow-growing, but the question asks why the ecosystem fails to regenerate, and the decisive factor is nutrient availability.
• D is wrong: exotic invasion is a consequence of disturbance, not the reason recovery is slow, and rain forest soil is not fertile in the first place.

Core concept: nutrient cycling and soil fertility in tropical ecosystems, linked to shifting cultivation and deforestation, a recurring Mains theme under environment and agriculture.

Trap: choosing C because slow-growing species sound like a convincing reason. The question asks about the ecosystem's cause, so look for the systemic factor.