Amorphous boron on burning in air forms:A. B(OH)_{ 3 } B. A mixture of B_{ 2 }O_{ 3 } and BNC. Only B_{ 2 }O_{ 3 } D. Only BN

When amorphous boron burns in air, it reacts with oxygen to form boron oxide. This is a combustion reaction where boron combines with oxygen.

The balanced chemical equation for the combustion of boron is:

\[ 4B + 3O_2 \rightarrow 2B_2O_3 \]

Here, boron burns in the presence of oxygen to form **boron(III) oxide (B₂O₃)**.

So, the correct answer is:

**C. Only B₂O₃**

Explanation of other options:
- **A. B(OH)₃**: Boric acid is formed when boron compounds, like B₂O₃, react with water, but this is not a direct product of boron burning in air.
- **B. A mixture of B₂O₃ and BN**: BN (boron nitride) is formed when boron reacts with nitrogen, not oxygen. So, this is incorrect.
- **D. Only BN**: BN would form in a nitrogen atmosphere, but since the reaction is in air (with oxygen), B₂O₃ forms instead.