Hypo is used in photography to

Options

A. reduce AgBr grains to metallic silver
B. convert metallic silver to silver unit
C. remove undecomposed silver bromide as a soluble complex
D. remove reduced silver

Explanation

To answer the question regarding the use of hypo in photography, we can follow these steps:

### Step-by-Step Solution:

1. **Identify Hypo**:
- Hypo is a common name for sodium thiosulfate, which has the chemical formula Na2S2O3.

2. **Understand the Role of Hypo in Photography**:
- In traditional photography, especially with film, silver bromide (AgBr) is used as a light-sensitive material. When exposed to light, it forms a latent image.

3. **Reaction of Hypo with Silver Bromide**:
- When the photographic film is developed, hypo (sodium thiosulfate) is used to react with the undeveloped silver bromide on the film. The reaction can be represented as:
\[ \text{Na}_2\text{S}_2\text{O}_3 + \text{AgBr} \rightarrow \text{Na}_3\text{AgS}_2\text{O}_3 + \text{NaBr} \]
- In this reaction, hypo reacts with silver bromide to form a soluble complex (sodium silver thiosulfate) and sodium bromide.

4. **Significance of the Reaction**:
- The formation of this soluble complex allows for the removal of undecomposed silver bromide from the film, which is crucial for the development process. This step helps in clearing the film of any unreacted silver bromide, thus ensuring that only the developed image remains.

5. **Conclusion**:
- Therefore, hypo is used in photography to remove undecomposed silver bromide as a soluble complex, making option C the correct answer.