Answer:
The correct option is d) an ecotone
Explanation:
An ecotone can be described as an area of transition between two different biomes. An estuary can be described as an ecotone where the river water meets the seawater. The freshwater biome mixes with the seawater biome in an estuary. Unique plants and animals are grown in an estuary which are adapted to live in such an environment. The water present in an estuary is termed as brackish water. Hence, the correct option is d.
