Lichens are used as the indicators of air pollution. These structures grow on tree bark and rocks. Air pollutants which are in the gaseous forms get dissolved in the rainwater, especially SO$ _2$ (sulphur dioxide) can damage lichens, and check them from growing. In places where lichens are not growing, it indicates that the air is heavily polluted with sulphur dioxide and other pollutants. 

Lichens are the symbiotic association of the algae and fungi. These absorb their mineral nutrients from the atmosphere rather than the soil. So, the contamination of the atmospheric air will result in their death. Lichens show specific sensitivity to sulphur dioxide and vanish from the area wherever it is present. So, these are called the indicators of the sulphur dioxide pollution.

Coliform bacteria are found in human faeces. Presence of such bacteria (E. coli is a member of the faecal coliform group) in surface water is a common indicator of faecal contamination. Sanitation program test detects the water for clearing contamination of faeces. Cholera vibrio, Salmonella typhi, Entamoeba histolytica are the pathogens found in the stagnant water or when the medical waste contaminates the water body. 

Lichens represent the symbiotic association between the algae and the fungi. These absorb their mineral nutrition from the moisture-laden air rather than the soil. So, the contamination of the air severely impairs their growth and they disappear from the areas that have a heavy concentration of sulphur pollutants in the air. Due to this sensitivity, these are considered to be the bioindicators of air pollution.

Lichen represents the symbiotic association between the algae and fungi. These absorb their mineral nutrients from the atmospheric moisture rather than the soil. So, when the air is contaminated with the sulphur oxides, they show extreme sensitivity to the toxic gas. Their existence is threatened by the oxides of sulphur in the air. The disappearance of the lichen, when exposed to the sulphur oxide, is used as bioindicator of the sulphur pollution in the air. 

Lichens are sensitive to the levels of SO$ _2$ in the atmosphere.

Lichens represent the symbiotic association between the algae and the fungi. These absorb their mineral nutrition from the moisture-laden air rather than the soil. So, the contamination of the air severely impairs their growth and they disappear from the areas that have a heavy concentration of sulphur pollutants in the air. Due to this sensitivity, these are considered to be the bioindicators of air pollution.

An indicator species is the one that either shows sensitivity or sustenance in the pollution. The lichens are the indicator species of air pollution because they cannot survive in the air polluted with the oxides of sulphur, released by industrial emissions. This is so because lichens absorb mineral nutrients from the moisture in the air and any contaminant in the air makes it difficult for them to perform the vital activities of life. Biston betularia is a peppered moth. Their population was dominated by white coloured varieties before the industrialization in England because the predators fed on black moths easily visible on the white coloured bark of the trees. Due to the industrialization, the pollution through emission released soot that was deposited on the white bark of the trees, due to which the white population shifted to black population as the white moths were now eaten by the predators.

MPCB is the Maharashtra Pollution Control Board. It implements a range of environmental legislation in the state of Maharashtra, India. The MPCB functions under the administrative control of Environment Department of the Government of Maharashtra.

Lichens represent the symbiotic association between the species of Algae and Fungi. These forms absorb much of their mineral nutrients from the air rather than the soil. So, any contamination would threaten their existence. If air is polluted with sulphur dioxide, then lichens will die in that particular environment. Since the era of industrialization, many species of lichens have gone extinct. Due to this property, lichens are considered to be the indicators of the air pollution. Hence, the correct answer is Usnea (Lichen)'

Lichens are the symbiotic associations between the algae and fungi. These absorb the mineral nutrients from the moisture-laden air instead of the soil. So, any contamination in the air is going to severely affect their growth. The oxides of sulphur have more adverse on the growth of lichens, These disappear from the area when sulphides are in excess in the air. So, these act as bioindicators to the air pollution caused by the sulphur oxides.

Insecta plecoptera (stonefly) is a sensitive insect that produces larvae in the water while the adults live on the land. The stonefly larvae are the indicators to the water pollution caused by the increase in the biological oxygen demand and the rise in temperature. Whenever there is an increase in the organic mass, the dissolved oxygen will decrease and the high rate of respiration of the algal bloom would also increase the temperature. Both these conditions are intolerable by the stonefly larvae and they die quickly. They need a high concentration of dissolved oxygen and normal temperature to survive.

The plants that are used as the indicators to the pollutants are very sensitive to the pollutants such that they either develop disease symptoms or cease to grow when these specific pollutants are present in the air, water or soil. For example, Salvia and Pinus are sensitive to ozone and sulphur dioxide pollution and they show red spots and curling of the leaves. Petunia and Chrysanthemum are sensitive to the peroxy-acetyl nitrate and they show the destruction of the chlorophyll. Lichens fail to grow in the sulphur dioxide pollution.

Stoneflies are the aquatic insects that also live on land as adults. These lay their eggs in water and the larvae hatch in the water itself. These larvae are highly sensitive to water pollution as they need a high concentration of dissolved oxygen to respire. They also show sensitivity to the temperature and as the temperature rises, they begin to die. So, when the water is polluted and has low dissolved oxygen and high temperature due to decomposition, the stoneflies will not be seen. Similarly, overgrowth of sewage fungi and sludge worms in the water body indicates a high concentration of organic pollution. Bloodworms are the endoparasite in the human body. 

Lichens are the most suitable bio-indicators for air pollution, especially sulfur dioxide pollution since they derive their water and essential nutrients mainly from the atmosphere rather than from the soil.  If the air is clean, shrubby, hairy and leafy lichens become abundant. A few lichen species can tolerate quite high levels of pollution and are commonly found on pavements, walls and tree bark in urban areas. The most sensitive lichens are shrubby and leafy while the most tolerant lichens are all crusty in appearance.

Escherechia coli is present in the intestines and is released with the faecal matter. Their presence in the water is indicative of the sewage mixup in the water source.

A. Hardness of water is indicated by the mineral ions concentration.

B. Industrial pollution is indicated by the presence of heavy metals, toxins etc.

C. Escherechia coli presence indicates sewage pollution in water.

D. Chlorine is added for the treatment or purification of water.

Hence, the correct answer is 'Sewage Pollution'.