Answer:
The correct answer is d. an ecotone
Step-by-step explanation:
An ecotone is the area of transition between two different biomes so ecotone is the boundary between two different habitats. In this area, two different communities meet with each other for example estuary are water bodies that are found at the place where streams of rivers meet the sea.
Therefore at estuary two ecosystems and their community meet with each other and the transition of river in sea occurs in this area. So estuary is transition phase of fresh water biome to salt water biome therefore the right is d. an ecotone.