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, and can be used as bioindicators of air quality.
Bryophytes, ferns, and algae are also sensitive to air pollution, but they are not as sensitive as lichens. Bryophytes are a group of non-vascular plants that include mosses, liverworts, and hornworts. Ferns are vascular plants that have large, feathery leaves. Algae are a diverse group of photosynthetic organisms that include single-celled organisms and multicellular organisms.
Sulfur dioxide is a gas that is released into the atmosphere by burning fossil fuels, such as coal and oil. It can also be released by volcanic eruptions. Sulfur dioxide can cause a variety of health problems, including respiratory problems, heart problems, and cancer. It can also damage plants and animals.
Lichens are very sensitive to sulfur dioxide because they have a very thin layer of cells that absorb water and nutrients from the air. Sulfur dioxide can damage this layer of cells, making it difficult for the lichen to survive.
Bryophytes, ferns, and algae are also sensitive to sulfur dioxide, but they are not as sensitive as lichens. This is because they have a thicker layer of cells that protects them from the effects of sulfur dioxide. However, if levels of sulfur dioxide are high enough, these plants can also be damaged.
Lichens are therefore the best indicator of sulfur dioxide pollution. They are very sensitive to the gas and can be used to monitor air quality.