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 determine which metal will not be deposited at the cathode during the electrolysis of their aqueous salts, we need to analyze the reduction potentials of the metals involved. The metal with the lowest reduction potential will not be deposited at the cathode because it has a lower tendency to gain electrons compared to the others.

### Step-by-Step Solution:

1. **Understand the Process of Electrolysis**:
- During electrolysis, reduction occurs at the cathode (gain of electrons) and oxidation occurs at the anode (loss of electrons).

2. **Identify the Metals and Their Reduction Potentials**:
- We need to look at the reduction potentials for the metals in question. The given reduction potentials are:
- Magnesium (Mg²⁺ + 2e⁻ → Mg): -2.36 V
- Copper (Cu²⁺ + 2e⁻ → Cu): +0.34 V
- Mercury (Hg²⁺ + 2e⁻ → Hg): +0.854 V
- Silver (Ag⁺ + e⁻ → Ag): +0.799 V

3. **Compare the Reduction Potentials**:
- The higher the reduction potential, the greater the tendency of the ion to gain electrons and be reduced.
- In this case, magnesium has the lowest reduction potential (-2.36 V), indicating that it is the least likely to be reduced and deposited at the cathode.

4. **Conclusion**:
- Since magnesium has the lowest reduction potential, it will not be deposited at the cathode during the electrolysis of its aqueous salt. Instead, it will undergo oxidation.

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