What is the main reason for thermal pollution?

The correct answer is option 3. Hot water released by power plants and industries.

Thermal pollution primarily occurs due to the release of hot water into natural water bodies, such as rivers or lakes, by power plants and industrial facilities. The heated water disrupts the natural temperature balance of the aquatic ecosystem, adversely affecting aquatic organisms and their habitats. This can lead to various ecological problems, including changes in species composition, reduced oxygen levels, and altered nutrient cycling.