Density gradient pycnocline layer.

• A. Layer with constant density
• B. Layer with no oxygen
• C. Layer where density changes rapidly with depth
• D. Surface mixed layer

Answer: (C)

The key idea is a density gradient in the water column. A pycnocline is the layer where density changes rapidly with depth, typically because temperature drops and/or salinity increases as you go down. This sharp change separates lighter surface waters from denser deep waters and helps limit vertical mixing. The surface mixed layer is relatively uniform in density due to wind and wave mixing, while a layer with no oxygen is about chemical composition, not how density varies with depth. A layer with constant density would not have a noticeable density gradient.