The sodium compound which is used for softening hard water:

If water has a high mineral concentration and contains calcium and magnesium salts, it is said to be hard. Water absorbs magnesium and calcium carbonates, sulfates, and bicarbonates when it percolates through limestone, chalk, or gypsum deposits.
Because soap reacts with the calcium in hard water, leaving a film on our hands after washing (instead of creating lather), it is necessary to rinse our hands more vigorously.

Hard water is the one in which it is difficult to make a lather with soap due to the presence of calcium and magnesium in the water.
Now, the removal of the calcium and magnesium ions from water is called the softening of water and this can be achieved by adding washing soda (Na₂CO₃) to water. It has the power to eliminate both transient and enduring harshness. Due to its insoluble nature in water, it causes a significant discharge of carbonate ions into the water. A precipitate of calcium carbonate is created when these carbonate ions interact with the dissolved calcium ions.

Hence, the correct option is A.