The correct answer is option 'C' - SO2 (sulfur dioxide).

Sulfur dioxide is a common pollutant emitted from metallurgical smelters, thermal power plants, and cement plants. It is produced during the combustion of fossil fuels that contain sulfur, such as coal and oil.

Below are the details explaining why SO2 is the common pollutant emitted from these industries:

1. Metallurgical Smelters:
Metallurgical smelters are facilities that extract metals from their ores through high-temperature processes. These processes involve the use of fossil fuels as a source of heat and reducing agents. When the fossil fuels are burned, sulfur compounds present in the fuel, such as sulfur-containing minerals, are oxidized to form sulfur dioxide. The sulfur dioxide is then emitted into the atmosphere.

2. Thermal Power Plants:
Thermal power plants are large facilities that generate electricity by burning fossil fuels, such as coal, oil, and natural gas. These fuels contain sulfur compounds, and when they are burned, sulfur dioxide is released as a byproduct of combustion. The emission of sulfur dioxide from power plants is a significant environmental concern due to its contribution to air pollution and its detrimental effects on human health and the environment.

3. Cement Plants:
Cement plants are industrial facilities that produce cement, a key material used in construction. The production of cement involves the heating of limestone and other raw materials in a kiln. Fossil fuels, such as coal and petroleum coke, are commonly used as a source of heat in the kiln. During the combustion of these fuels, sulfur dioxide is formed as a result of the sulfur content in the fuel. The emitted sulfur dioxide can contribute to air pollution and acid rain formation.

Sulfur dioxide is considered a harmful pollutant due to its various adverse effects. When released into the atmosphere, it can react with other compounds to form sulfuric acid, a major component of acid rain. Sulfur dioxide can also contribute to the formation of particulate matter and smog, which can have detrimental effects on human health, particularly respiratory health.

Therefore, the common pollutant emitted from metallurgical smelters, thermal power plants, and cement plants is sulfur dioxide (SO2).