The correct answer is - D. Climate.
The Amazon rainforest is located on and in close proximity of the Equator. Because of its location, the Amazon has a climate that is consistent and has no changes throughout the year. Its climate is defined by being relatively warm, and there's rainfall almost every day of the year. These conditions allow the plants to be able to grow throughout of the year because they have the same conditions for all of the year. That has contributed to very dense and fast growing vegetation in the Amazon, which makes it the place that has the highest biodiversity in the whole world.
