The Mathura Refinery situated close to the Taj Mahal is a threat to the marble of the Taj Mahal because the Refinery releases the air pollutants in the form of oxides of sulphur, oxides of nitrogen and oxides of carbon. These oxides then react with water and precipiate in the form of nitric acid and sulphuric acid called acid rains. Acid rains have resulted in the corrosion of the marble, making it to appear yellow. Hence, the correct answer is 'Air Pollutants from Mathura Refinery'.