When $C{O}_2$  gas is passed through lime water, it turns milky, Which of the following compounds is responsible for this milkiness ?

When carbon dioxide gas is passed through calcium hydroxide or lime water, it reacts with the water to form Carbonic acid(H₂CO₃) first. This carbonic acid then reacts with Calcium hydroxide and forms Calcium carbonate, which is responsible for the milkiness.

 H₂CO₃+Ca(OH)₂ → CaCO₃+2H₂O