Which of the following does not exist in free form?

Options

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

Explanation

To determine which of the given compounds does not exist in free form, we need to analyze each option individually:

1. **BF3 (Boron Trifluoride)**:
- BF3 is a well-known inorganic compound that exists as a colorless gas. It is commonly used as a Lewis acid in various chemical reactions. Therefore, BF3 does exist in free form.

2. **BCl3 (Boron Trichloride)**:
- BCl3 is another inorganic compound that exists as a colorless gas. It is used as a reagent in organic synthesis. Thus, BCl3 also exists in free form.

3. **BBr3 (Boron Tribromide)**:
- BBr3 is a colorless fuming liquid that contains boron and bromine. It is known to exist, although it may appear red-brown due to bromine contamination. Hence, BBr3 exists in free form.

4. **BH3 (Boron Hydride)**:
- BH3, or boron hydride, does not exist as a stable free molecule under normal conditions. Instead, it tends to dimerize to form B2H6 (diborane) or exists in other complex forms. Therefore, BH3 does not exist in free form.

Based on the analysis, the compound that does not exist in free form is **BH3**.

### Step-by-Step Solution:
1. Analyze BF3: Exists as a colorless gas and is a Lewis acid.
2. Analyze BCl3: Exists as a colorless gas and is used in organic synthesis.
3. Analyze BBr3: Exists as a colorless fuming liquid, though may appear red-brown.
4. Analyze BH3: Does not exist in free form; it dimerizes to form B2H6.

### Final Answer:
**BH3 does not exist in free form.**