Boron can be obtained by various methods but not by :

Options

A. thermal decomposition of `B_(2)H_(6)`
B. pyrolysis of `BI_(3)` (van Arkel)
C. reducing `BCl_(3) " with " H_(2)`
D. electrolysis of fused `BCl_(3)`

Answer

To determine which method cannot be used to obtain boron, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Thermal Decomposition of B2H6 (Diborane)**:
- The reaction for the thermal decomposition of diborane (B2H6) can be represented as:
\[ 2 B_2H_6 \rightarrow 4 B + 6 H_2 \]
- This indicates that boron can be obtained from diborane.

2. **Pyrolysis of BCl3 (Boron Trichloride)**:
- The pyrolysis of boron triiodide (Bi3) can be represented as:
\[ 2 BCl_3 \rightarrow 2 B + 3 Cl_2 \]
- This shows that boron can also be obtained from BCl3.

3. **Reducing BCl3 with H2**:
- The reduction of boron trichloride (BCl3) with hydrogen (H2) can be represented as:
\[ BCl_3 + 3 H_2 \rightarrow B + 3 HCl \]
- This indicates that boron can be obtained through this method as well.

4. **Electrolysis of Fused BCl3**:
- The electrolysis of fused boron trichloride (BCl3) does not yield boron. Instead, boron is typically obtained via the electrolysis of fused potassium tetrafluoroborate (KBF4):
\[ KBF_4 \rightarrow B + K + F_2 \]
- Therefore, this method does not produce boron.

### Conclusion:
Based on the analysis, boron can be obtained through thermal decomposition of diborane, pyrolysis of boron triiodide, and reducing BCl3 with hydrogen. However, boron cannot be obtained by the electrolysis of fused BCl3.

Thus, the correct answer is:
**Boron cannot be obtained by electrolysis of fused BCl3.**