`{:(,"Column A",,,"Column B"),(A.,"Removal of hardness by boiling","( )",a.,"Resins"),(B.,"Clark's method","( )",b.,"Can remove temporary and permanent hardness"),(C.,"Addition of washing soda","( )",c.,"Decomposition reaction"),(D.,"Ion exchange method","( )",d.,"Addition of slaked lime"):}`

Explanation

To solve the matching question between Column A and Column B regarding the removal of water hardness, we will analyze each item in Column A and find the corresponding match in Column B step by step.

### Step-by-Step Solution:

1. **Removal of hardness by boiling (A)**:
- Boiling water removes temporary hardness. This is because bicarbonates present in the water decompose into carbonates upon heating, leading to precipitation.
- **Chemical Reaction**:
\[ \text{Ca(HCO}_3\text{)}_2 \xrightarrow{\text{heat}} \text{CaCO}_3 \downarrow + \text{CO}_2 + \text{H}_2\text{O} \]
- **Match**: This process is a decomposition reaction, so A matches with C (Decomposition reaction).

2. **Clark's method (B)**:
- In Clark's method, slaked lime (calcium hydroxide) is added to water to remove temporary hardness.
- **Chemical Reaction**:
\[ \text{Ca(OH)}_2 + \text{Ca(HCO}_3\text{)}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{Ca(HCO}_3\text{)}_2 \]
- **Match**: Since slaked lime is added, B matches with D (Addition of slaked lime).

3. **Addition of washing soda (C)**:
- Washing soda (sodium carbonate) can remove both temporary and permanent hardness by precipitating calcium and magnesium ions.
- **Chemical Reaction**:
\[ \text{CaCl}_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{CaCO}_3 \downarrow + 2\text{NaCl} \]
- **Match**: This can remove both temporary and permanent hardness, so C matches with B (Can remove temporary and permanent hardness).

4. **Ion exchange method (D)**:
- The ion exchange method uses ion exchange resins to remove undesirable ionic contaminants from water.
- **Match**: The solid used in this method is called ion exchange resins, so D matches with A (Resins).

### Final Matching:
- A → C (Decomposition reaction)
- B → D (Addition of slaked lime)
- C → B (Can remove temporary and permanent hardness)
- D → A (Resins)