The correct answer is - Habitat.
The shallow and brackish water in an estuary represents the habitat that the estuary is providing for the living organisms in it.
The reason why this is a description of the habitat of the estuary is that the description provides information about the living conditions in it. It tells us what kind of waters there, and the water is the basis for the existence of the marine life. By the description, it also can tell us what kind of a marine life can be expected to live there, as certain species are well adapted for this kind of habitat, while others are not.
