The correct option is B discoloring white marbles of Taj Mahal.
Since the area in which Taj Mahal is located is surrounded by many industries and factories, they emit a great amount of pollutants into the atmosphere. They even release toxic and acidic components in the nearby water bodies. The pollutants in air are discolouring the white marbles of Taj Mahal due to acidic corrosion (caused by the oxides of non-metals like sulfur and nitrogen).