What is an enclosed body of coastal water that is a mixture of saltwater and fresh water called?
a. watershed
b. tributary
c. drainage basin
d. estuary

An estuary is an enclosed body of coastal water that is a mixture of saltwater and freshwater. This type of body of water provides a transition zone between river environments and maritime environments. This body of water is subject to both marine influences (e.g. tides, waves, salt water) and river influences (e.g. flow of fresh water and sediment) which provide high levels of nutrients in the water and in the soil. Estuaries are known to be one of the most productive natural habitats of different organisms on Earth.

Physical systems is the answer