Option 1: Na2CO3 (Sodium Carbonate) - This is the correct answer. Sodium carbonate is commonly used for softening hard water. It works by reacting with the minerals that cause hardness, such as calcium and magnesium ions, and converting them into insoluble compounds that can be easily removed from the water.

Option 2: NaHCO3 (Sodium Bicarbonate) - Sodium bicarbonate is not typically used for softening hard water. It can be used as a temporary solution to raise the pH of water, but it does not effectively remove the minerals that cause hardness.

Option 3: NaOH (Sodium Hydroxide) - Sodium hydroxide is a strong base and is not commonly used for softening hard water. It is more commonly used for cleaning and industrial purposes.

Option 4: Na2SO4 (Sodium Sulfate) - Sodium sulfate is not used for softening hard water. It is a salt that can be found naturally in some water sources, but it does not have the ability to remove the minerals that cause hardness.