The correct answer is option 3: Thermal pollution. Thermal pollution refers to the addition of excessive amounts of heat to a lake or any body of water. This can occur when industrial processes or power plants release heated water into nearby bodies of water, causing a rise in temperature. The increase in temperature can have negative effects on the ecosystem of the lake, leading to changes in the behavior and metabolism of aquatic organisms, as well as reduced dissolved oxygen levels. It can also disrupt the natural balance of the lake, affecting the growth of algae and other aquatic plants. It is important to note that while the other options may be related to environmental issues, they do not specifically refer to the addition of excessive heat to a lake.