Exp) Option d is the correct answer.

Option d is correct. Environmental Protection Agency (EPA)’s national ambient air quality standards for SO2 are designed to protect against exposure to the entire group of sulfur oxides (SOx).   The largest sources of SO2 emissions are from fossil fuel combustion at power plants and other industrial facilities.  

Source: “https://indianexpress.com/article/upsc-current-affairs/upsc-essentials/upsc-essentials-weekly-news-express-rains-air-quality-subansiri-icelands-earthquakes-and-more-key-takeaways-and-points-to-ponder-9025983/

https://www.epa.gov/so2-pollution/sulfur-dioxide-basics#:~:text=The%20largest%20source%20of%20SO,plants%20and%20other%20industrial%20facilities.”

https://science.nasa.gov/earth/climate-change/greenhouse-gases/indias-growing-sulfur-dioxide-emissions/