`Na_2S_2O_3. 5H_2O` is used in photography to

Options

A. Reduce AgBr to metallic Ag
B. Remove reduced Ag
C. Remove undecomposed AgBr as a soluble complex
D. Convert metallic Ag to silver salt

Answer

To solve the question regarding the use of `Na2S2O3.5H2O` in photography, we will break it down step by step.

### Step 1: Identify the compound
The compound `Na2S2O3.5H2O` is known as sodium thiosulfate. It is often used in various chemical processes, including photography.

**Hint:** Remember the chemical name of the compound, as it is crucial for understanding its applications.

### Step 2: Understand its role in photography
In photography, particularly in the development of photographic films, there are silver halides (like AgBr) that are used to capture images. After the exposure, these films contain undecomposed silver bromide (AgBr) that needs to be removed to avoid unwanted darkening.

**Hint:** Think about what happens to undeveloped photographic films and the need to process them.

### Step 3: Reaction with silver bromide
Sodium thiosulfate reacts with the undecomposed silver bromide (AgBr) in the photographic film. It reduces AgBr to metallic silver (Ag) and forms a soluble complex, which can then be washed away easily.

**Hint:** Focus on the chemical interaction between sodium thiosulfate and silver bromide.

### Step 4: Conclusion on its function
The primary function of sodium thiosulfate in photography is to remove undecomposed AgBr as a soluble complex, allowing for the clearing of the film and revealing the developed image.

**Hint:** Summarize the main function of sodium thiosulfate in the context of photographic processing.

### Final Answer
Therefore, `Na2S2O3.5H2O` is used in photography to remove undecomposed AgBr as a soluble complex.

**Correct Option:** C