(c) Sodium carbonate decahydrate (Na₂CO₃.10H₂O)
The textbook (Section 2.4.3) explicitly states that one of the uses of washing soda (Na₂CO₃.10H₂O) is "for removing permanent hardness of water." It reacts with soluble calcium and magnesium salts causing hardness, converting them into insoluble carbonates. The other options are incorrect: NaHCO₃ is baking soda (antacid/fire extinguisher), NaCl is common salt, and CaSO₄.½H₂O is Plaster of Paris.
Source: Chapter 2, Section 2.4.3