Correct answer: (a) Lichens
Explanation
- 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.
- B
Methyl mercury
Methyl mercury is a toxic pollutant (Minamata), not a living bioindicator organism. It is the poison, not the indicator species.
- C
Rose plant
A rose plant is not the textbook bioindicator of atmospheric pollution in this list.
- 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.
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