The correct answer is (b), halocline.
A halocline is a layer in a body of water where the water density changes rapidly with depth due to a change in salinity. The salinity of water is a measure of its salt content. The salt content of water can vary greatly depending on the source of the water. For example, ocean water is much saltier than freshwater.
The halocline is important because it can affect the movement of water in the ocean. The denser water at the bottom of the ocean is less likely to mix with the less dense water at the top of the ocean. This can create a barrier that prevents nutrients and other substances from being transported from the bottom of the ocean to the top.
The halocline can also affect the distribution of marine life. Many marine animals are adapted to live in specific salinity ranges. The halocline can create barriers that prevent these animals from moving from one area to another.
(a) Thermocline is a layer in a body of water where the temperature changes rapidly with depth.
(c) Pycnocline is a layer in a body of water where the density changes rapidly with depth.
(d) Chemocline is a layer in a body of water where the chemical composition changes rapidly with depth.