The correct answer is C — they have a fixed number of molecules of water of crystallisation in one formula unit of these salts.
"Water of crystallisation" means the fixed number of water molecules chemically bonded in one formula unit of a salt crystal (e.g., Na₂CO₃·10H₂O, CuSO₄·5H₂O, CaSO₄·2H₂O, FeSO₄·7H₂O). This is the one property common to all four salts. They are not all coloured (washing soda is white), nor are they all acidic or basic — they are salts, which can be neutral, acidic, or basic depending on the parent acid/base.