The correct option is A Due to the formation of calcium carbonate.
Limewater is calcium hydroxide solution (Ca(OH)2) and it is a base. Carbon dioxide (CO2) being a non-metallic oxide (acidic in nature), reacts with the calcium hydroxide to form a calcium carbonate (CaCO3). The reaction involved is given below:
Calcium carbonate obtained is white and insoluble in water, hence it makes limewater appear milky.