The correct answer is: B. Epilimnion
The epilimnion is the upper layer of a body of water that is warmer and less dense than the water below it. It is typically well-mixed by wind and currents, and supports most of the aquatic life in the lake.
The hypolimnion is the lower layer of a body of water that is colder and denser than the water above it. It is typically not well-mixed, and does not support much aquatic life.
The thermocline is a layer of water in a lake or ocean where the temperature changes rapidly with depth. It is typically located between the epilimnion and hypolimnion.
The hydroline is the boundary between the water column and the atmosphere. It is located at the surface of the water.
Here is a diagram that illustrates the different layers of a lake:
The epilimnion is the warm, well-mixed layer of water at the top of the lake. The hypolimnion is the cold, dense layer of water at the bottom of the lake. The thermocline is the layer of water between the epilimnion and hypolimnion where the temperature changes rapidly with depth.