A white sublimable substance, that turns black on treatment with an `NH_(3)` solution can be:

Options

A. `Hg_(2)Cl_(2)`
B. `HgCl_(2)`
C. `As_(2)O_(3)`
D. `NH_4Cl`

Answer

To solve the problem of identifying a white sublimable substance that turns black upon treatment with an ammonia solution, we will analyze the given compounds one by one.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are:
- Hg2Cl2 (Mercury(I) chloride)
- HgCl2 (Mercury(II) chloride)
- As2O3 (Arsenic trioxide)
- Ammonium chloride (NH4Cl)

2. **Analyze Hg2Cl2**:
- When Hg2Cl2 is treated with an ammonia solution (NH3), it forms HgNH2Cl, which is a white precipitate. Additionally, elemental mercury (Hg) is produced, which is black.
- **Conclusion**: Hg2Cl2 can turn black when treated with ammonia.

3. **Analyze HgCl2**:
- When HgCl2 is treated with ammonia, it also forms a white precipitate (HgNH2Cl). However, no black compound is formed in this reaction.
- **Conclusion**: HgCl2 does not turn black when treated with ammonia.

4. **Analyze As2O3**:
- When As2O3 is treated with ammonia, it forms ammonium arsenate (NH4AsO4), which is also white. No black compound is produced.
- **Conclusion**: As2O3 does not turn black when treated with ammonia.

5. **Analyze Ammonium Chloride (NH4Cl)**:
- When ammonium chloride is treated with ammonia, there is no reaction that produces a black compound. The ions remain in solution without forming any precipitate.
- **Conclusion**: Ammonium chloride does not turn black when treated with ammonia.

6. **Final Conclusion**: The only compound that meets the criteria of being a white sublimable substance that turns black upon treatment with ammonia is **Hg2Cl2**.

### Answer:
The correct answer is **Hg2Cl2 (Mercury(I) chloride)**.