The correct answer is (b) Lichens.
Lichens are a symbiotic relationship between a fungus and an alga or cyanobacterium. They are very sensitive to air pollution, especially sulfur dioxide. When exposed to high levels of sulfur dioxide, lichens can become discolored, stunted, or even die. This makes them a good indicator of air quality.
Bryophytes are non-vascular plants that include mosses, liverworts, and hornworts. They are also sensitive to air pollution, but they are not as sensitive as lichens. Ferns are vascular plants that have roots, stems, and leaves. They are not as sensitive to air pollution as bryophytes or lichens. Algae are photosynthetic organisms that can be found in both aquatic and terrestrial environments. They are not as sensitive to air pollution as bryophytes, lichens, or ferns.
In conclusion, lichens are the best indicator of sulfur dioxide pollution because they are the most sensitive to this type of pollution.