Which one of the following is correctly matched ?

The correct match is Pycnocline – Sea water density. In oceanography, various "clines" represent layers where specific physical properties change rapidly with depth:

• Pycnocline: A layer in a body of water where the density increases rapidly with depth.
• Thermocline: A layer where temperature changes (usually decreases) rapidly with depth. Option C is incorrect because it specifies air temperature.
• Halocline: A layer where salinity changes significantly with depth. Option D is incorrect as it links halocline to temperature.
• Ecocline: A transition zone between two ecosystems or communities along an environmental gradient (such as moisture or altitude). While it involves changes in species, it is defined by the environmental gradient rather than biodiversity itself.