Question

saltwater and freshwater mix. marsh grasses grow. shellfish, birds, and others live here. what am i? lake river ocean estuary

Explanation:

Estuaries are areas where saltwater (from the ocean) and freshwater (from rivers, lakes) mix. They are characterized by the growth of marsh grasses and serve as habitats for shellfish, birds, and many other organisms. Lakes are inland bodies of water, rivers are flowing watercourses, and oceans are large salt - water bodies. None of these have the specific characteristic of being a mix of saltwater and freshwater with marsh grasses and the mentioned diverse life forms like estuaries do.

Answer:

Estuary