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.

The answer to your question is C because ...

The open ocean is the sunlit top layer of the ocean beyond the continental shelves. The vast bulk of the sea, it covers over 360,000,000 square kilometres of the Earth's surface. It's referred to as the 'marine desert' because nutrients are lower here than in the shallow seas and life can be scarce.