The correct answer is option 4: Hypolimnion. The hypolimnion refers to the lower layer of water in a lake that is typically deeper and colder compared to the upper layers. This layer usually remains stagnant and receives less sunlight, resulting in lower oxygen levels. The hypolimnion is separated from the upper layers by the thermocline, which is a distinct boundary in the water column. The hypolimnion is important for various ecological processes as it provides habitat for cold-water fish species and supports decomposition of organic matter. It is also responsible for maintaining the stratification and stability of the lake`s water column.