Which one of the following compound is a gas (at `0^(@)C`):

Options

A. `BF_(3)`
B. `BCl_(3)`
C. `BBr_(3)`
D. `Bl_(3)`

Explanation

To determine which of the following compounds is a gas at \(0^\circ C\), we can analyze the four trihalides of boron provided: BF3, BCl3, BBr3, and BI3.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds in question are:
- BF3 (Boron trifluoride)
- BCl3 (Boron trichloride)
- BBr3 (Boron tribromide)
- BI3 (Boron triiodide)

2. **Understand the Trend in Van der Waals Forces**: As we move down the group in the periodic table (from F to Cl to Br to I), the size of the halogen atoms increases. This increase in size leads to an increase in the surface area of the molecules, which in turn increases the van der Waals forces of attraction between the molecules.

3. **Evaluate the State of Each Compound at \(0^\circ C\)**:
- **BF3**: Being the smallest and having the least van der Waals forces due to its small size, BF3 is a gas at \(0^\circ C\).
- **BCl3**: Larger than BF3, BCl3 has stronger van der Waals forces and is a gas at room temperature but may not be gaseous at \(0^\circ C\).
- **BBr3**: Even larger, BBr3 has even stronger van der Waals forces and is likely to be a liquid at \(0^\circ C\).
- **BI3**: The largest of the four, BI3 has the strongest van der Waals forces and is a solid at \(0^\circ C\).

4. **Conclusion**: Based on the analysis of van der Waals forces and molecular size, the compound that exists as a gas at \(0^\circ C\) is **BF3**.

### Final Answer:
**BF3** is the compound that is a gas at \(0^\circ C\).