The correct answer is: B. Washing Soda
Permanent hardness of water is caused by the presence of calcium and magnesium salts that are not precipitated by boiling. These salts can be removed by adding washing soda (sodium carbonate), which reacts with the calcium and magnesium ions to form insoluble precipitates. The precipitates can then be filtered out of the water.
Chlorine, potassium permanganate, and bleaching powder are all disinfectants that can be used to kill bacteria in water. However, they will not remove permanent hardness.
Washing soda is a white, crystalline solid that is used in a variety of applications, including water treatment. It is a base, and it reacts with acids to form salts. In the case of permanent hardness, washing soda reacts with the calcium and magnesium ions to form insoluble precipitates of calcium carbonate and magnesium carbonate. These precipitates can then be filtered out of the water, leaving the water soft.
Washing soda is a safe and effective way to remove permanent hardness from water. It is available at most hardware stores.