Correct Answer: Formation of lather with soap

Solution : The correct option is the Formation of lather with soap .

To determine the hardness of water, the lather or soap test is widely employed. Hard water combines with soap to generate soap scum or curd, an intractable material. When you use soap in hard water, it produces less lather and instead forms a scum. This can make cleaning harder, as more soap is needed to produce the desired cleansing effect. Soft water, on the other hand, readily forms a lather with soap.