The metal that cannot obtained by electrolysis of an aqueous solution of its salts is `:`

Options

A. `Cu`
B. `Cr`
C. `Ag`
D. `Ca`

Explanation

To solve the question of which metal cannot be obtained by electrolysis of an aqueous solution of its salts, we will analyze the reduction potentials of the given metals: copper, chromium, silver, and calcium.

### Step-by-Step Solution:

1. **Understanding Electrolysis**:
- Electrolysis involves the reduction of cations at the cathode. The metal that can be deposited depends on the reduction potentials of the metal ions compared to that of water.

2. **Reduction Potentials**:
- We need to compare the reduction potentials of the metals with that of water. The standard reduction potential for water is approximately -0.83 V.
- The reduction potentials for the given metals are:
- Calcium (Ca²⁺): -2.87 V
- Chromium (Cr³⁺): +0.52 V
- Copper (Cu²⁺): +0.79 V
- Silver (Ag⁺): +1.33 V

3. **Comparison with Water**:
- A metal ion will only be reduced at the cathode if its reduction potential is higher than that of water.
- In this case, we compare:
- Calcium: -2.87 V (less than -0.83 V)
- Chromium: +0.52 V (greater than -0.83 V)
- Copper: +0.79 V (greater than -0.83 V)
- Silver: +1.33 V (greater than -0.83 V)

4. **Identifying the Metal**:
- Since calcium has a reduction potential of -2.87 V, which is much lower than that of water (-0.83 V), it cannot be reduced during electrolysis.
- The other metals (chromium, copper, and silver) have higher reduction potentials than water and can be reduced to their metallic forms.

5. **Conclusion**:
- Therefore, the metal that cannot be obtained by electrolysis of an aqueous solution of its salts is **Calcium (Ca)**.

### Final Answer:
The correct answer is **Calcium (Ca)**.