When carbon dioxide is passed through lime water, it turns lime water milky. Which of the following compound is formed during this reaction? 

Calcium carbonate

The correct answer is option 2. Calcium carbonate.

When carbon dioxide is passed through lime water (aqueous calcium hydroxide), it reacts to form calcium carbonate. The chemical reaction is represented as follows:

\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]

This reaction results in the formation of calcium carbonate, which is only slightly soluble in water. The calcium carbonate formed during the reaction is responsible for turning the lime water milky or forming a white precipitate. Therefore, the correct compound formed in this reaction is Calcium carbonate (CaCO₃)