In which location is it most likely that salinity of the seawater will be the highest? a. in polar regions b. along the cost where rivers enter the ocean c. at the surface of the open ocean d. in tropical regions

I suspect that the answer is at the surface of the open ocean. This is where the evaporation of water takes place which concentrates the salts at the very surface. However, with any significant mixing, the effect might not be measureable. Nevertheless, the surface tension of the water at the surface would tend to stabilize the very surface slowing down mixing with more dilute water just beneath the surface.

It is nonrenuable which means that once you have used it, it's gone forever.