Which one of the following is correctly matched?

1. A

   Ecocline – Biodiversity

   Option (a) claims that Ecocline represents Biodiversity, which is incorrect as an ecocline is actually a zone of gradual transition between different ecosystems rather than biodiversity itself.

2. B

   Pycnocline – Sea water density

   Option (b) claims that Pycnocline represents Sea water density, which is the correct match because a pycnocline is a layer in an ocean or other body of water in which water density increases rapidly with depth.

3. C

   Thermocline – Air temperature

   Option (c) claims that Thermocline represents Air temperature, which is incorrect because a thermocline is a distinct layer in a large body of water where temperature changes more rapidly with depth than it does in the layers above or below.

4. D

   Halocline – Sea water temperature

   Option (d) claims that Halocline represents Sea water temperature, which is incorrect because a halocline is a subtype of chemocline caused by a strong, vertical salinity gradient in a body of water.

Summary. Official key is (b) Pycnocline – Sea water density. This question tests the geographical and oceanographic terms related to vertical gradients in water bodies. A pycnocline specifically denotes the zone of rapid density change in sea water. Similarly, thermocline corresponds to temperature and halocline corresponds to salinity in aquatic environments. Only option (b) provides the accurate pairing among the choices given.