The correct answer for the question "Which one of the following is used for water softening?" is option 3, Na6P6O18.

Water softening is the process of removing minerals, such as calcium and magnesium ions, from water. These minerals are responsible for the hardness of water. In order to soften water, a water softener is used, which contains a substance called a water softening agent.

In this question, the options are different compounds, and we need to identify which one is used for water softening.

Option 1, Mg(HCO3): This compound is magnesium bicarbonate. While it contains magnesium, it is not commonly used as a water softening agent. Alert - correct answer should be MgSO4 (magnesium sulfate).

Option 2, Na3PO: This compound is sodium phosphate. It is not typically used for water softening.

Option 3, Na6P6O18: This compound is sodium hexametaphosphate. It is a commonly used water softening agent. It works by binding to calcium and magnesium ions in the water, preventing them from causing hardness.

Option 4, Na2HPO: This compound is sodium hydrogen phosphate. It is not commonly used for water softening