The correct answer is: A. Alum
Permanent hardness of water is caused by the presence of calcium and magnesium salts. These salts are not removed by boiling water, and they can cause problems with soap scum and scale buildup. Alum is a chemical compound that can be used to remove permanent hardness from water. When alum is added to water, it reacts with the calcium and magnesium salts to form a precipitate. This precipitate can then be filtered out of the water, leaving the water soft.
Sodium carbonate, lime, and potassium permanganate are not effective in removing permanent hardness from water. Sodium carbonate is a base, and it can react with the calcium and magnesium
salts to form a precipitate. However, this precipitate is not as effective at removing the hardness as alum. Lime is a base, and it can also react with the calcium and magnesium salts to form a precipitate. However, this precipitate is even less effective at removing the hardness than sodium carbonate. Potassium permanganate is an oxidizer, and it can react with the calcium and magnesium salts to form a precipitate. However, this precipitate is also not as effective at removing the hardness as alum.In conclusion, the only option that is effective in removing permanent hardness from water is alum.