The correct answer is:
c.lake
A lake would have the least variation in its salinity, whereas salt marsh, mangrove forest, and estuary wouldn't.
Step-by-step explanation:
Salinity is the saltiness or number of salt dissolved in a body of water. The weathering of rocks releases minerals, including salt, into the ocean. Evaporation of ocean water and the production of sea ice both double the salinity of the ocean. However, these "salinity raising" parts are frequently counterbalanced by processes that lower salinity such as the continuous input of fresh water from rivers, presumption of rain and snow, and melting of ice.