The correct option is D Power plants
Contamination of air by unwanted substances that have harmful effect on environment is called air pollution. These unwanted substances are called pollutants. Pollutants released from factories, dung cakes and powerplants are the sources of air pollution.
Forests are not contributor of air pollution. Infact forests play an important role in purifying the air by giving us oxygen for survival and reducing the level of carbon dioxide in atmosphere by using it for photosynthesis.