A thermocline is a thin zone of rapid change in water ____.

• A. Temperature
• B. Salinity
• C. Density
• D. Depth

Answer: (A)

A thermocline is a thin layer where water temperature changes rapidly with depth. In stratified waters, a warm surface layer sits above cooler depths, and the temperature drops quickly across a relatively narrow vertical span, creating the thermocline. The other options describe gradients in salinity (halocline) or density (pycnocline), or are not a property gradient at all, so the defining feature here is the sharp temperature change.