Taj Mahal is mainly threatened by the deleterious effects of:

The correct answer is Sulphur dioxide.
Important Points

Acid Rain:

• Acid rain is a byproduct of a variety of human activities that emit the oxides of sulphur and nitrogen in the atmosphere.
• It refers to how acid from the atmosphere is deposited on the earth’s surface.
• Oxides of nitrogen and sulphur which are acidic in nature can be blown by the wind and finally settle down either on the ground as dry deposition or in water, fog and snow as wet deposition.
• Acid rain is harmful to agriculture, plants as it dissolves and washes away nutrients needed for their growth. It causes respiratory ailments in human beings, animals as well as the aquatic ecosystem.
• It can even be damaging to buildings and other structures made of stone or metal.
• For example, the air around the city of Agra, where the Taj Mahal is located, contains fairly high levels of sulphur and nitrogen oxides, due to a large number of industries and power plants around the area.
• The resulting acid rain reacts with marble, CaCO3 of Taj Mahal (CaCO3 +H2SO4 → CaSO4 + H2O+ CO2) causing damage to this monument leading it to get discoloured and lustreless.