Which one of the following denotes water salinity gradient?

The correct answer is B) Halocline. A halocline is a zone in a water body where salinity changes sharply with depth. The suffix -cline means a steep gradient layer, and the prefix tells you which property is changing. Here that property is salt content.

Why the others are wrong:

• Thermocline (A): a layer of rapid temperature change. Temperature, not salinity.
• Pycnocline (C, printed here as Pyconocline): a layer of rapid density change. Density depends on both temperature and salinity, so it is a broader gradient, not a salinity gradient.
• Chemocline (D): a layer where chemical composition changes sharply, typically in meromictic lakes where substances like sulphide build up at depth. It is about chemistry, not salt content.

Concept link: vertical stratification controls nutrient mixing, marine productivity and thermohaline circulation. This comes up in Geography (oceanography, Indian Ocean salinity) and gives a Mains angle on climate change and fisheries.

Trap: candidates pick thermocline because it is the most familiar cline, and forget that the question asks specifically about salinity, not temperature or density.