Which of the following is used for removing the permanent hardness of water?

Washing soda

The correct answer is option 4. Washing soda.

Washing soda, also known as sodium carbonate \((Na_2CO_3)\), is commonly used for removing permanent hardness of water through a process called precipitation. When washing soda is added to hard water containing calcium and magnesium ions, it reacts with these ions to form insoluble calcium carbonate \((CaCO_3)\) and magnesium carbonate \((MgCO_3)\), which precipitate out of solution. This reduces the concentration of calcium and magnesium ions in the water, thereby removing the hardness.