The gas which causes yellowing of Taj Mahal
A. ${H_2}S$
B. $S{O_2}$
C. $C{O_2}$
D. $N{O_2}$

Hint: Taj Mahal is made up of white marble which is a metamorphic rock containing calcite which is made by limestone or calcium carbonate. Due to increasing air pollution, the colour of Taj Mahal is changing from white to yellow. The gas causing the yellowing of Taj Mahal must be one that can react with a base, here calcium carbonate.

Complete step by step answer:
Air contains a variety of gases namely nitrogen, oxygen, carbon dioxide, etc. Normally they are present in the appropriate proportion and the balance of nature is maintained. But with the increasing emissions from industries and factories of gases like carbon dioxide, Sulphur oxides and nitrogen oxides, air pollution increases.
The gas causing the yellowing of Taj Mahal is Sulphur dioxide. When Sulphur in the chimneys of factories reacts with atmospheric oxygen, it forms Sulphur dioxide and escapes into the atmosphere. It reacts with the water molecules present in the atmosphere forming sulphurous acid and sulphuric acid. These acids form precipitate in the form of acid rain.
 $S(s) + {O_2}(g) \to S{O_2}(g)$
$S{O_2} + {H_2}O \to {H_2}S{O_3}$
$S{O_3} + {H_2}O \to {H_2}S{O_4}$
The acid rain digests and destroys the outer layer of Taj Mahal thus changing its colour and making cracks in the monument as well. The acids present dissolve the calcium carbonate to form calcium sulphate as a result of an acid base neutralization reaction causing the corrosion of the marble and turning it yellow.
$CaC{O_3} + {H_2}S{O_4} \to CaS{O_4} + C{O_2} + {H_2}O$

Hence, the correct option is B.

Note:
The government has been taking various steps to prevent the corrosion of Taj Mahal by reducing air pollution. Factories near Taj Mahal are constantly kept under check regarding the gaseous emissions from their chimneys. Taj Mahal is among the Seven Wonders of the World and a common tourist attraction. Corrosion of the monument is a great loss for both the historical culture and the tourism of India.