Which of the following acid reduce `AgNO_3` into `Ag`.

Options

A. `H3PO_2`
B. `H_3PO_4`
C. `H_4P_2O_7`
D. `H_3PO_3`

Answer

To determine which acid can reduce AgNO3 to Ag, we need to analyze the given acids and their reducing properties.

### Step-by-Step Solution:

1. **Identify the Acids**: The acids provided are:
- H3PO2 (Phosphorous acid)
- H3PO4 (Phosphoric acid)
- H4P2O7 (Pyrophosphoric acid)
- H3PO3 (Phosphorous acid)

2. **Understand the Reducing Agents**: A reducing agent is a substance that donates electrons to another substance, reducing it while itself getting oxidized. Among the given acids, we need to identify which one has strong reducing properties.

3. **Analyze Each Acid**:
- **H3PO2**: This is a known reducing agent, but it's not as strong as H3PO3.
- **H3PO4**: This is a strong acid but does not act as a reducing agent.
- **H4P2O7**: This is a derivative of phosphoric acid and does not exhibit reducing properties.
- **H3PO3**: This is a well-known reducing agent and can reduce AgNO3 to Ag.

4. **Chemical Reaction**: The reaction can be represented as:
\[ 3AgNO_3 + H_3PO_3 + 3H_2O \rightarrow 3Ag + 3HNO_3 + H_3PO_4 \]
In this reaction, H3PO3 donates electrons to AgNO3, reducing it to metallic silver (Ag).

5. **Conclusion**: The acid that reduces AgNO3 to Ag is H3PO3 (Phosphorous acid).

### Final Answer:
The correct answer is **H3PO3**.