A biome is a set of ecosystems in a delimited area containing characteristic groups of plant and animal species.
An example of a biome is the desert. In this biome, the plant species are mostly cacti and the animal species are specially adapted to hot climates with little water. Other biome examples are tundra and chaparrals.
On the other hand, the estuary is not a biome but the mouth of a river that joins the sea. Therefore, the answer is D.