The refers to the region below the mixed layer where salinity changes as you move deeper underwater.

Halocline

Explanation:

Refers to the salt and slope is a subtype of chemocline. That causes strong sharp salinity changes as you move deep down the water in oceans.  A in the middle latitudes there is an increase of vertical stratification and hence the waters being warmers than the deep and shallow cooler waters such regions ate prone to the mixing of sea salts. Found in fjords and mixing estuaries, at about 160 feet the water temperature reaches -1.5 degrees which are a nearly freezing point. Another point is that halocline is commonly found in areas of higher limestone contents such as the caves.

Then the answer would be d then