`{:(,"Column A",,,"Column B"),(A.,"Hard water","( )",a.,"Reverse osmosis"),(B.,"Soft water","( )",b.,"Rain water"),(C.,"Desalinated water","( )",c.,"Presence of "Ca^(+2)andMg^(2+)),(D.,"Lake water","( )",d.,"Scum formation"),(,,"( )",e.,"Lather formation"):}`

Explanation

To solve the matching question between Column A and Column B, we will analyze each item in Column A and find the corresponding match in Column B.

### Step-by-Step Solution:

1. **Identify Hard Water**:
- Hard water is characterized by the presence of calcium ions (Ca²⁺) and magnesium ions (Mg²⁺).
- It also leads to scum formation when soap is used.
- **Match**: Hard water (A) → Presence of Ca²⁺ and Mg²⁺ (C) and Scum formation (D).

2. **Identify Soft Water**:
- Soft water is known for its ability to form lather easily with soap, making it suitable for washing.
- **Match**: Soft water (B) → Lather formation (E).

3. **Identify Desalinated Water**:
- Desalinated water is produced through processes like reverse osmosis, which removes salts and impurities from seawater.
- **Match**: Desalinated water (C) → Reverse osmosis (A).

4. **Identify Lake Water**:
- Lake water often contains a significant amount of rainwater, as lakes are typically fed by rain and runoff.
- **Match**: Lake water (D) → Rain water (B).

### Final Matches:
- A. Hard water → C. Presence of Ca²⁺ and Mg²⁺
- B. Soft water → E. Lather formation
- C. Desalinated water → A. Reverse osmosis
- D. Lake water → B. Rain water

### Summary of Matches:
- A → C (Presence of Ca²⁺ and Mg²⁺)
- B → E (Lather formation)
- C → A (Reverse osmosis)
- D → B (Rain water)