Sulphur dioxide (SO2) is responsible for turning the Taj Mahal yellow. When fossil fuels, such as coal and oil, are burned they release pollutants into the air, including sulphur dioxide. This gas reacts with oxygen and moisture in the atmosphere to form sulphuric acid, which then falls onto the surface of the Taj Mahal. The acid interacts with the calcium carbonate in the marble, causing a chemical reaction that results in the yellowing of the monument. Option 1 (Carbon dioxide) and option 2 (Nitrogen) are not responsible for turning the Taj Mahal yellow. While option 3 (Chlorine) is known to cause damage, it is not specifically associated with the yellowing of the Taj Mahal. Therefore, the correct answer is option 4 (Sulphur dioxide).