The largest cotton producing state in India is Gujarat. Cotton dust is defined as dust present in the air during the handling or processing of cotton, which may contain a fiber, bacteria, fungi, soil, pesticides and other contaminants which may have accumulated with the cotton during the growing, harvesting and subsequent processing or storage periods and causes hazardous effects on health while inhaling. So, cotton dust is an important pollutant here.

Carbon-dioxide is responsible for 'Global-Warming.
A layer of $CO _2$ in the atmosphere traps the infrared radiation coming from earth's surface. These trapped radiations heat the earth's atmosphere which results in green house effect and global warming.

The noise from the horn and engines of the vehicles results in sound pollution. The use of extensive chemical fertilizers causes accumulation of these chemicals in the soil and leaching into the ground water. The runoff of these chemicals into water causes water pollution. The improper disposal of sewage results in water pollution. The use of CFC-free refrigerator does not cause any air pollution.

The air pollution is basically of two types:
A. Gaseous pollution: Harmful gases given out from variety of sources. 
B. Particulate pollution: Particles such as those of dust, smoke, mining, stone drilling, wearing of rubber tyres of motor vehicles.

 In traffic-congested city air, nitrogen oxides produce brown air or reddish-brown haze. The pollutant that does not get degraded or broken down into the harmless material is a nondegradable pollutant example Benzene Hexachloride (BHC). Anthropogenic pollution is man-made pollution resulting from human activities like burning fossil fuel, deforestation, mining, sewage etc. A primary air pollutant is a pollutant that persists in the environment in the form it is produced example carbon monoxide.

Secondary air pollutants are usually produced photochemically from primary pollutants and are called photochemical oxidants. Secondary air pollutants occur in photochemical smog. Nitrogen oxides, oxygen and ketone bodies combine to form peroxy acyl nitrates or PAN which is a major constituent of photochemical smog. PAN damages chloroplasts, inhibit electron system and spoil enzyme systems controlling cellular metabolism which causes Hill reaction during photosynthesis .

The primary pollutants are directly released from the source like hydrocarbons are released when fossil fuels are burnt and nitrogen oxides are released due to vehicular combustion and also fossil fuel burning. When these primary pollutants react with each other they form a more potent secondary pollutant called the ozone, which is a poisonous gas in the troposphere.

Primary pollutants include ammonia, sulfur dioxide, nitrogen dioxide and carbon monoxide. Secondary pollutants include ground-level ozone, acid rain and nutrient enrichment compounds.

The photochemical smog has the chief constituent of the peroxy-acetyl nitrate (PAN) that are secondary pollutants as they formed by the primary pollutants. These are formed when the hydrocarbon radicals react with the oxygen in the presence of the sunlight. It forms a brown-coloured haze. This also results in the formation of the ground-level ozone. 

• Primary pollutants are the pollutants that are directly released or emitted in the atmosphere. 
• Secondary pollutants are the pollutants which are formed when primary pollutants react with themselves or with other components of atmosphere.

Secondary pollutant is an air pollutant that do not emit directly from the source but forms when primary pollutant reacts in the atmosphere. For example $O _3$ and PAM forms when hydrocarbon reacts with nitrogen oxides in the presence of sunlight, smog forms when industrial or vehicle fumes reacts with sunlight in atmosphere. Thus, the correct answer is 'All of the above'.

Secondary pollutants are pollutants which form in the atmosphere. These pollutants are not emitted directly from a source (like vehicles or power plants). Instead, they form as a result of the pollutants emitted from these sources reacting with molecules in the atmosphere. Secondary pollutants are concerning as they can be formed from many different compounds. Photochemical smog is made up of various secondary pollutants like ozone, peroxyacyl nitrates (PANs), and nitric acid.

Biodegradable pollutants become part of rapid turnover in biogeochemical cycles. The examples of biodegradable pollutants are sewage, garbage, animal waste and livestock. Most non-biodegradable pollutants do not enter biogeochemical cycle e. g., DDT, BHC, plastics, polyethylene, cans, glass, etc.

Dichlorodifluoromethane (R-12) is a colorless gas usually sold under the brand name Freon-12. Thus, the chemical name of freon is chlorodifluoromethane. It is used as a refrigerant and aerosol spray propellant. 

Secondary pollutants are the pollutants formed by the chemical reaction between primary pollutants in the presence of sunlight e.g., sulphuric acid, nitrogen dioxide and ozone. 

Synergism means the process of acting together. In pollution, the synergism is seen when the primary pollutants are less harmful but they combine to form a more potent secondary pollutant. For example, the presence of ozone in the troposphere is undesired and this ozone has come due to the combination of hydrocarbons and nitrogen oxides (both are primary pollutants).

a. Nitrous oxide is formed as a result of denitrification in which the nitrates are converted to oxides of nitrogen and released into the atmosphere.