Acid rain is one of the major cause for discoloring of Taj Mahal.
True
Taj Mahal is made up of Marble, which contains Calcium, that is basic in nature. The rainwater turns acidic due to Sulphur and Nitrogen based pollutants, forming Sulphurous and Nitrous acids. In the case of Acid rain, acid in the rainwater reacts with this material and causes discolouring.