A lagoon can be a type of estuarine habitat, specifically a coastal lagoon, where freshwater from rivers or streams mixes with saltwater from the sea, resulting in brackish water. The term "estuary" is a broad classification that includes various coastal water bodies, such as lagoons, bays, and river mouths.

B. River: is a freshwater habitat (or the freshwater source for an estuary) and does not typically contain the mix of high-salinity ocean water that defines an estuarine environment, except at its very end where it meets the sea.

C. Lakes: are generally freshwater habitats (though some rare landlocked lakes can be saline) and do not involve the mixing of saltwater with freshwater due to tidal action.

D. Ocean: is a marine (saltwater) habitat with high salinity, distinct from the brackish, partially enclosed conditions of an estuary.