Out of the following matals that cannot be obtained by electrolysis of the aquenous solution of their salts is

Options

A. Cr
B. Na
C. Ca
D. Mg

Explanation

To solve the question of which metal cannot be obtained by electrolysis of its aqueous salt solution, we will analyze the given options: chromium, sodium, calcium, and magnesium.

### Step-by-Step Solution:

1. **Understanding Electrolysis**:
- Electrolysis involves breaking down a compound into its constituent elements using electricity. In an aqueous solution, the metal ions (M^n+) and anions (X^m-) are present. At the cathode, reduction occurs, where metal ions gain electrons to form solid metal (M^0).

2. **Identify the E° Values**:
- The ability of a metal to be deposited at the cathode during electrolysis depends on its standard reduction potential (E°). If the E° of the metal ion is lower than that of water, the metal cannot be deposited.

3. **Analyzing Each Metal**:
- **Chromium (Cr)**:
- Chromium is a d-block element with an atomic number of 24. It is not metallic in nature in its common oxidation states in aqueous solutions, and thus it cannot be deposited through electrolysis.
- **Sodium (Na)**:
- Sodium is highly electropositive with a high E° value. It can be deposited from its aqueous solution.
- **Calcium (Ca)**:
- Calcium, like sodium, is also highly electropositive. However, its E° value is lower than that of water, meaning it cannot be deposited through electrolysis of its aqueous solution.
- **Magnesium (Mg)**:
- Magnesium is also highly electropositive and can be deposited from its aqueous solution.

4. **Conclusion**:
- Among the options, chromium cannot be obtained by electrolysis of its aqueous salt solution due to its non-metallic nature. Calcium also cannot be deposited due to its E° value being lower than that of water. However, the question asks for the metal that cannot be obtained by electrolysis, and since chromium is not metallic, it is the correct answer.

### Final Answer:
**Chromium cannot be obtained by electrolysis of its aqueous salt solution.**