Which among the following is a commonly used bioindicator of air pollution?

Lichen is a composite organism that comprises both fungi and algae. They are often found growing on the bark of trees, rocks, and soil. Lichens are extremely sensitive to air pollutants, such as sulfur dioxide, nitrogen oxides, and heavy metals, making them a commonly used bioindicator of air pollution. 

Therefore, lichen is the correct answer. Mushrooms, leeches, and viruses are not commonly used as bioindicators of air pollution.