The correct answer is A. Hydrogen sulphide.
Lead acetate is a white solid that dissolves in water to form a colorless solution. When lead acetate solution is exposed to hydrogen sulfide, it forms a black precipitate of lead sulfide. This is a chemical reaction that can be used to test for the presence of hydrogen sulfide.
Carbon dioxide, methane, and oxygen are all gases that are not likely to react with lead acetate. Therefore, if a paper moistened with lead acetate turns black when placed in a manhole, it is likely that the sewer contains hydrogen sulfide.
Hydrogen sulfide is a colorless, flammable gas with a rotten egg smell. It is produced by the decomposition of organic matter, such as sewage. Hydrogen sulfide is toxic and can cause a number of health problems, including respiratory problems, nausea, and vomiting.
If you are exposed to hydrogen sulfide, it is important to seek medical attention immediately.