Option 1 claims both statements are correct and that II explains I. This is false as tropical rainforest soil is not nutrient-rich - nutrients are quickly recycled by the ecosystem due to rapid decomposition, they don`t remain in the soil.

Option 2 also states both statements are correct, but II is not an explanation for I. This is false like option 1, because Statement I is incorrect: the soil in tropical rainforests is not rich in nutrients.

In Option 3, it is stated that Statement I is correct and II is incorrect. This isn`t true. As previously noted, the soil in tropical rainforests isn`t nutrient-rich, hence I is incorrect. However, statement II is correct as dead organic matter decomposes quickly due to high temperature and moisture.

Option 4 is the correct answer. Statement I is incorrect as explained before, but Statement II is correct. The high temperature and moisture in tropical rain forests facilitate the rapid decomposition of dead organic matter. Thus, even though there is high biological activity, the nutrients don`t accumulate in the soil but instead in the biomass.