The correct answer is the first option: the soil of a rainforest is deficient in nutrients.

Tropical rainforests have some of the most nutrient-poor soils in the world. The high temperatures and heavy rainfall in these forests cause rapid weathering and leaching of minerals from the soil, leading to a deficiency of nutrients such as nitrogen, phosphorus, and potassium. This means that the trees and other plants in the rainforest have adapted to survive in nutrient-poor conditions, but they cannot grow as quickly as the trees in a tropical deciduous forest, which typically have more nutrient-rich soil.

The other options listed are not accurate explanations for why tropical rainforests do not regenerate as quickly as tropical deciduous forests. Propagules of trees in a rainforest typically have high viability, and rainforest species can be fast-growing, depending on the species. Exotic species may invade disturbed rainforest areas, but this is not a major factor in the slow regeneration of these forests.