Lichens indicate pollution of

Answer: Option B

Lichens indicate pollution of SO₂.
A lichen is a composite organism that emerges from algae or cyanobacteria living among the filaments of fungi in a mutually beneficial symbiotic relationship. The fungi benefit from the carbohydrates produced by the algae or cyanobacteria via photosynthesis.
Many lichens are very sensitive to environmental disturbances and can be used to cheaply assess air pollution, ozone depletion and metal contamination.
The sensitivity of a lichen to air pollution is directly related to the energy needs of the mycobiont, so the stronger the dependency of the mycobiont on the photobiont, the more sensitive the lichen is to air pollution.