The correct answer is Carbon Dioxide. When air is blown from the mouth into limewater, it turns milky due to the presence of Carbon Dioxide (CO2). This is because the exhaled breath contains a significant amount of CO2. When CO2 comes in contact with limewater, a chemical reaction occurs resulting in the formation of Calcium Carbonate (CaCO3), which gives the milky appearance.

Option 1, Water vapour, is incorrect as it doesn`t react with limewater to produce a milky solution.

Option 2, Oxygen, is also incorrect. Although oxygen is present in exhaled air, it doesn`t react with limewater.

Option 4, Carbon Monoxide, is incorrect as well. Exhaled breath does not usually contain carbon monoxide unless the person has been inhaling it, and it also doesn`t react with limewater to create a milky solution.