Answer : 2. "जल की स्थायी कठोरता को दूर करने के लिए"
Explanation :

It is commonly known as Na2CO3 or sodium carbonate. It is used in the softening of hard water i.e. it is used to remove the temporary and permanent hardness of the water. It is commonly used as a cleaning agent for domestic purposes. It is also used in the manufacturing of paper, soap, textiles, paints, etc.