Which one of the following is the well known example of bloindicator of air pollution?

1. A

   Lichens

   Lichens are the classic bioindicators of air quality, especially sulphur dioxide. They are sensitive symbiotic associations of fungus and alga/cyanobacterium; many species vanish from polluted air. That is why (a) is the official key.

2. B

   Methyl mercury

   Methyl mercury is a toxic pollutant (Minamata), not a living bioindicator organism. It is the poison, not the indicator species.

3. C

   Rose plant

   A rose plant is not the textbook bioindicator of atmospheric pollution in this list.

4. D

   Sunflower

   Sunflower is used in some phytoremediation talk, not as the well-known air-pollution bioindicator here.

Summary. Official key is (a) lichens. They signal SO2 and general air quality. Methyl mercury is a toxin, not an indicator organism; rose and sunflower are not the standard pair.