The correct answer is: A. Carbon dioxide
Lime water is a solution of calcium hydroxide in water. When carbon dioxide is present in the air, it reacts with the calcium hydroxide to form calcium carbonate, which is a white precipitate. This is why lime water turns milky when it is exposed to air.
Nitrogen, oxygen, and sulfur dioxide are all gases that are present in the air. However, they do not react with calcium hydroxide in the same way that carbon dioxide does. Therefore, they do not cause lime water to turn milky.
Here is a more detailed explanation of the reaction between calcium hydroxide and carbon dioxide:
Ca(OH)2(aq) + CO2(g) â CaCO3(s) + H2O(l)
In this reaction, the calcium hydroxide reacts with the carbon dioxide to form calcium carbonate and water. The calcium carbonate is a white precipitate, which is why lime water turns milky when it is exposed to carbon dioxide.
I hope this explanation is helpful! Let me know if you have any other questions.