The solubility of these salts in water depends on their ionic/covalent character and their solubility products ($K_{sp}$).

• $\text{Na}_2\text{S}$: As an alkali metal salt, it is highly ionic and highly soluble in water.
• $\text{ZnS}$ and $\text{CuS}$: Both are heavy metal sulphides and are sparingly soluble. However, according to Fajan's rules, $\text{Cu}^{2+}$ has a higher polarising power than $\text{Zn}^{2+}$, making $\text{CuS}$ more covalent and less soluble than $\text{ZnS}$. This is also consistent with qualitative analysis, where $\text{CuS}$ precipitates in Group II (acidic medium, requiring very low $S^{2-}$ concentration due to its extremely low $K_{sp} \approx 10^{-36}$), whereas $\text{ZnS}$ precipitates in Group IV (basic medium, requiring higher $S^{2-}$ concentration as its $K_{sp} \approx 10^{-24}$). Therefore, the correct decreasing order of solubility is $\text{Na}_2\text{S} > \text{ZnS} > \text{CuS}$.