An estuary is a. a freshwater biome b. an area with a low concentration of minerals and nutrients c. characterized by deep water d. an ecotone

Estuares are the areas where the rivers meets the sea. It is the transition area between the river and the sea. And that is by definition an ecotone. They are among the most productive ecosystems in the nature. The answer is d. An ecotone

Bwould be my guess |-/ 

It is c. naturally in the environment