Acoastal inlet or bay in which freshwater from rivers mixes with salty ocean water is called a(n) a. transition zone. b. abyssal plain. c. estuary. d. hydrothermal vent.

c. estuary

Explanation:

    An estuary is an area along the coast where a river joins the sea. The estuaries are partially closed, having a free connection to the open sea, this connection may be permanent or periodic. In the estuaries, seawater is diluted by freshwater, forming a brackish environment.

81 is a composite number.