Taj Mahal is being slowly disfigured and discoloured. This is primarily due to

We have to identify the main cause that is slowly disfiguring and discolouring the marble of the Taj Mahal. For such a monument, the chief culprit found by scientists has been the corrosive action of acidic pollutants present in the atmosphere.

First, recall that burning of coal, petroleum and other fossil fuels in the neighbouring industrial area (for example, the Mathura refinery and many small industries around Agra) releases large amounts of $$\text{SO}_2$$ (sulphur-dioxide) and $$\text{NO}_x$$ (oxides of nitrogen) into the air.

These gases dissolve in atmospheric moisture and undergo further oxidation to form the strong mineral acids $$\text{H}_2\text{SO}_4$$ and $$\text{HNO}_3$$. When it rains, the water droplets now contain these acids, and we call this phenomenon acid rain.

The marble of the Taj Mahal is mainly calcium carbonate $$\text{CaCO}_3$$. Acid rain reacts chemically with marble as follows:

$$\text{CaCO}_3 + \text{H}_2\text{SO}_4 \;\longrightarrow\; \text{CaSO}_4 + \text{CO}_2 + \text{H}_2\text{O}$$

The product $$\text{CaSO}_4$$ (calcium sulphate) is more soluble and easily washed away; consequently the smooth surface becomes rough, loses its shine and develops a yellowish or brownish colour. Continuous exposure causes pitting, flaking and loss of fine carvings—exactly the disfigurement observed on the Taj Mahal.

Now we look at each given option:

   • Global warming mainly raises temperature; it does not directly corrode marble.
   • Acid rain, as just explained, chemically attacks marble and causes the observed damage.
   • Water pollution concerns rivers and lakes, not the stone of the monument.
   • Soil pollution affects the ground but again does not account for the surface corrosion seen on the monument’s marble.

So the only option that matches the scientific explanation is acid rain.

Hence, the correct answer is Option B.