Right answer is (a) Hardening of steel
To explain I would say: Hardening of steel generates a hazardous waste i.e. Spent hardening salt, cyanide, nitrate and nitrate sludge which is very much hazardous to the environment and human health, so, it should be disposed of properly. But Hardening of rock salt, hardening of gold, hardening of coal doesn’t produce the wastes listed above because of different processing.