The correct answer is: B. Ordinary soap
Hard water contains dissolved minerals, such as calcium and magnesium, which can react with soap to form a scum. This scum is what makes it difficult to get a lather with soap in hard water.
Ordinary soap is made from fats or oils that have been combined with an alkali, such as sodium hydroxide or potassium hydroxide. This process is called saponification. When soap is added to hard water, the calcium and magnesium ions in the water react with the soap molecules to form insoluble salts. These salts are what cause the scum.
Detergent, on the other hand, is made from synthetic chemicals that are not affected by hard water. This is why detergents are often used in place of soap in hard water areas.
Liquid soap is simply soap that has been dissolved in water. It is no more or less effective at removing scum than solid soap.
Dilute acid can also be used to remove scum from hard water. However, this is not a recommended method, as it can damage the surfaces being cleaned.
In conclusion, hard water and soft water can be distinguished by using ordinary soap. Ordinary soap will form a scum when added to hard water, while detergent will not.