Threshold Limit Value (TLV)
The Threshold Limit Value (TLV) is a term commonly used in occupational health and safety to define the maximum allowable concentration of a hazardous substance in the air. It represents the level at which it is believed that nearly all workers may be repeatedly exposed day after day without adverse health effects. TLVs are established by reputable organizations such as the American Conference of Governmental Industrial Hygienists (ACGIH) and are based on current scientific knowledge and research.

Safe Limit for SO2 in Air
The safe limit for sulfur dioxide (SO2) in the air is defined by its TLV. SO2 is a colorless gas with a pungent odor that is produced by burning sulfur-containing fossil fuels, such as coal and oil. It is a major air pollutant that can have harmful effects on both human health and the environment.

The correct answer to the given question is option 'A', which is 5 ppm (parts per million). This means that the maximum allowable concentration of SO2 in the air, as determined by TLV, is 5 ppm.

Explanation
Sulfur dioxide can cause a range of health effects, including irritation of the respiratory system, exacerbation of asthma symptoms, and increased risk of respiratory infections. It can also contribute to the formation of acid rain and particulate matter pollution, which can have further environmental consequences.

The TLV for SO2 at 5 ppm is based on extensive research and consideration of the available scientific evidence. This value is set to protect workers from the potential health effects associated with prolonged exposure to SO2 in occupational settings.

It is important to note that TLVs are not intended to be absolute "safe" values, but rather represent concentrations that are believed to be reasonably safe for most workers. Individual susceptibility, exposure duration, and other factors can influence the actual risk associated with exposure to a particular substance.

In conclusion, the safe limit for sulfur dioxide (SO2) in the air, as determined by its TLV, is 5 ppm. This value is based on scientific research and represents the maximum allowable concentration that workers can be exposed to without adverse health effects.