The metal which will not be deposited at cathode during electrolysis of their aq. salts.

Options

A. Ag and Cu
B. Hg and Cu
C. Mg
D. Cu, Hg and Ag

Explanation

To solve the question of which metal will not be deposited at the cathode during the electrolysis of their aqueous salts, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Ions Present**:
In the electrolysis of aqueous salts, the metal ions and hydrogen ions (H⁺) are present. For the given metals, we have:
- Ag⁺ (Silver ion)
- Cu²⁺ (Copper ion)
- Hg²⁺ (Mercury ion)
- Mg²⁺ (Magnesium ion)

2. **Reference the Electrochemical Series**:
The electrochemical series ranks elements based on their tendency to gain electrons (be reduced). The lower the metal ion is in the series, the more readily it will be reduced and deposited at the cathode.

3. **Analyze Each Metal Ion**:
- **Ag⁺**: Silver is low in the electrochemical series and will be deposited at the cathode.
- **Cu²⁺**: Copper is also low in the electrochemical series and will be deposited at the cathode.
- **Hg²⁺**: Mercury is similarly low in the series and will be deposited at the cathode.
- **Mg²⁺**: Magnesium is higher in the electrochemical series compared to H⁺. Therefore, during electrolysis, H⁺ will be reduced and deposited at the cathode instead of Mg²⁺.

4. **Conclusion**:
Since Mg²⁺ cannot be reduced in the presence of H⁺ (which is lower in the electrochemical series), it will not be deposited at the cathode during the electrolysis of its aqueous salt. Therefore, the answer is **Mg**.

### Final Answer:
The metal which will not be deposited at the cathode during electrolysis of their aqueous salts is **Mg** (Magnesium).