Majorly, lichens are the pollution indicators of

The correct option is SO₂ (Sulfur Dioxide).

Lichens are complex organisms that arise from a symbiotic relationship between a fungus (mycobiont) and an alga or cyanobacterium (photobiont). Unlike vascular plants, lichens do not have a well-developed cuticle, root system, or stomata. Instead, they absorb water, nutrients, and gases directly from the atmosphere across their entire surface area.

Because of this high absorption rate and lack of filtering mechanisms, lichens are extremely sensitive to air pollutants, especially sulfur dioxide (SO₂). Exposure to SO₂ disrupts their photosynthetic processes by breaking down chlorophyll, which ultimately leads to the death of the photobiont and the lichen itself. Consequently, areas with high levels of sulfur dioxide pollution typically show a significant decline or total absence of lichens, making them excellent biological indicators (bioindicators) of SO₂ pollution levels.