An organism that has adapted to surviving in both salt water and fresh water would be best suited to survive in which aquatic ecosystem?

the intertidal zone

a river

an estuary

the open ocean

Option C, Estuary

Explanation:

An estuary is a geographical entity where the salty water of ocean mixes with fresh water of river. The resulting water is known as brackish water. The brackish water is not as salty as that of ocean and also the salinity level of water rise and fall regularly.

Thus, an organism that is able to live in both salty water and fresh water environment can survive in an estuary.

Hence, option C is correct