Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of chemical substances or energy, such as noise, heat or light.

The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water is called Biochemical Oxygen Demand(BOD)
                             
The amount of BOD in the water is a measure of the amount of organic material in the water, in terms of how much oxygen will be required to break it down biologically.

Pollutant is a substance that pollutes something, especially water or the atmosphere.
Examples are particulate matter (Sooty particles that are most toxic when they are small),sulphur dioxide,carbon-monoxide etc.

Primary pollutants are those gases or particles that are pumped into the air to make it unclean. They include carbon monoxide from automobile (cars) exhausts and sulphur dioxide from the combustion of coal.

Lead is a major air pollutant.Smelters (metal refineries) and other metal industries; combustion of leaded gasoline in piston engine aircraft; waste incinerators (waste burners), and battery manufacturing.

All the options are causes of industrial pollution
Asbestos:
This pollutant has cancer-causing properties. When inhaled, it can cause illnesses such as asbestosis and some types of cancer.
Nitrates & Phosphates:
These are found in fertilizers, and are often washed from the soils to nearby water bodies. They can cause eutrophication, which can be very problematic to marine environments.
Oils:
Oils form a thick layer on the water surface because they do not dissolve in water. This can stop marine plants receiving enough light for photosynthesis.

Land pollution is the deterioration (destruction) of the earths land surfaces, often directly or indirectly as a result of mans activities and their misuse of land resources.
It occurs when waste is not disposed of properly, or can occur when humans throw chemicals onto the soil in the form of pesticides, insecticides and fertilizers during agricultural practices.

Rain can carry and deposit the nitrogen in some pollutants on rivers and soils. This will adversely affect the nutrients in the soil and water bodies. This can result in algae growth in lakes and water bodies, and make conditions for other living organism harmful.this is called eutrophication.

Chemical reactions involving air pollutants can create acidic compounds which can cause harm to vegetation and buildings. Sometimes, when an air pollutant, such as sulfuric acid combines with the water droplets that make up clouds, the water droplets become acidic, forming acid rain.

Secondary pollutants: When pollutants in the air mix up in a chemical reaction, they form an even more dangerous chemical. Photochemical smog is an example of this, and is a secondary pollutant.

1. Population explosion ends up in over use of natural resources and hence pollutes the environment. For example higher population pollutes our water resources around us.
2. industralisation ends up in the release of effluents into water bodies or spoiling land areas.
3. Cutting down trees for fuel and burning end up in air pollution.
4. Urbanisation after deforestation ends up in polluted atmosphere and deficit of fresh water for human use.

Most of the chemicals that pollute the environment (mainly water) are man-made, resulted from various activities. Toxic chemicals used for various purpose are thrown into water. Organic and inorganic pollutants are the main causes of chemical pollution. For example, many rivers and surface water bodies were contaminated by numerous waste streams from various chemical plants.

Thermal pollution is the degradation of the quality of water due to a sudden change of its temperature. A primary source of thermal pollution is the discharge of heated water from the power plants. The unused heat, generally around $60$% of the input energy which is unable to be used in the process, is then returned to a sink, such as water.

Electrostatic precipitator is a device that uses an electric charge to remove certain impurities such as solid particles liquid  from air and ether gases. It is particularly used to remove particles and power generating stations. 

Forest fires and droughts are caused naturally as man is not responsible for these. 

Nitrates in water can cause severe illness in infants and domestic animals. Common sources of excess nitrate reaching lakes and streams include septic systems, animal feedlots, agricultural fertilizers, manure, industrial wastewaters, sanitary landfills, and garbage dumps. So D is the correct option. 

A person who is exposed to small amounts of insecticides over a long period of time can experience the effects of chronic poisoning and also pesticide spray can drift and pollute the air. Thus less use of insecticides should be done.

Deforestation increases the chances of natural calamities.
Deforestation means cutting down trees where it increases the chances of floods and droughts.
Note:
Deforestation may be caused naturally or there may be man made causes of deforestation.

Air pollution, water pollution and soil pollution represents types of environmental pollution. The contamination of air, water and soil is the environmental pollution. Certain impurities are added to environment due to human activities such as burning coal, dumping waste in water, deforestation, burning of fuel etc.

Coal fires, volcanoes and forest fires causes air pollution due to release of many pollutants such as $CO _2$, whereas High-speed wind and dust storm cause pollution.

All of the following are the consequences of deforestation.
(A) Decrease in rainfall
(B) Decrease in fertility of soil
(C) Increased global warming
Cutting of trees reduces amount of rainfall, it also decreases the fertility of soil. Deforestation increases the temperature and carbon dioxide level thus causing increased global warming.