The correct answer is A. Sodium Carbonate.
Permanent hardness of water is caused by the presence of calcium and magnesium salts that are not easily removed by boiling. These salts can form scale on pipes and appliances, and can also make soap less effective.
Sodium carbonate (Na2CO3), also known as washing soda or soda ash, is a chemical compound that can be used to remove permanent hardness from water. When sodium carbonate is added to water, it reacts with the calcium and magnesium salts to form a precipitate, which can then be filtered out.
Alum (Al2(SO4)3 · 14H2O), also known as potassium aluminum sulfate, is a chemical compound that can also be used to remove permanent hardness from water. However, alum is not as effective as sodium carbonate, and it can also leave a residue in the water.
Potassium permanganate (KMnO4) is a chemical compound that can be used to remove temporary hardness from water. However, it cannot be used to remove permanent hardness.
Lime (CaO) is a chemical compound that can be used to remove both temporary and permanent hardness from water. However, lime is not as effective as sodium carbonate, and it can also leave a residue in the water.