The correct answer to the question is option 2 - Lichen. Lichen is a unique organism that consists of a symbiotic relationship between a fungus and an alga or cyanobacteria. Lichen is known for its sensitivity towards air pollution. It can effectively monitor air quality by being highly responsive to changes in pollution levels. Lichen absorbs water and nutrients directly from the atmosphere, making it susceptible to air pollutants. When the air quality deteriorates, lichen undergoes changes in color, growth, or even dies. This makes lichen a reliable indicator of air pollution. Options 1, 3, and 4 - bacteria, algae, and fungi, are not specifically known for their ability to monitor or indicate air pollution.