Permanent hardness of water can be removed by which of the following compound?

Washing soda

The correct answer is option 4. Washing soda.

Uses of washing soda

(i) Sodium carbonate (washing soda) is used in glass, soap and paper industries.

(ii) It is used in the manufacture of sodium compounds such as borax.

(iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.

(iv) It is used for removing permanent hardness of water.