The tropical rain forests are hot and wet all year, hence the best answer as to why the vegetation remains evergreen is D (constant heat and abundant water).

The soils of the rain forest are actually not that fertile as most nutrients are quickly taken up by the lush vegetation or leached away by heavy rainfall. Therefore, choices B (rich soil...) and C (fertile soils...) do not work. Choice A does not work as there is high humidity in the rain forest.