Colloidal solutions of metals like gold can be prepared when their salt solutions react with certain substances like `SnCl_(2)` , formaldeyde, phenyl hydrazine , etc.
`2AuCl_(3) + 3SnCl_(2) to 3SnCl_(4)+ underset("sol")(2Au)`
The above method is an example of

Options

A. reduction method
B. oxidation method
C. hydrolysis method
D. double decomposition method

Explanation

To solve the question regarding the preparation of colloidal solutions of metals like gold through the reaction of their salt solutions with substances like SnCl₂, we will analyze the provided reaction step by step.

### Step-by-Step Solution:

1. **Identify the Reaction**:
The reaction given is:
\[ 2 \text{AuCl}_3 + 3 \text{SnCl}_2 \rightarrow 3 \text{SnCl}_4 + 2 \text{Au (sol)} \]
Here, gold (Au) is being produced from its chloride salt (AuCl₃).

2. **Determine the Oxidation States**:
- In AuCl₃, gold (Au) is in the +3 oxidation state.
- In the product, elemental gold (Au) has an oxidation state of 0.
- In SnCl₂, tin (Sn) is in the +2 oxidation state, and in SnCl₄, it is in the +4 oxidation state.

3. **Analyze the Changes in Oxidation States**:
- Gold (Au) is reduced from +3 to 0, indicating that it gains electrons.
- Tin (Sn) is oxidized from +2 to +4, indicating that it loses electrons.

4. **Identify the Type of Reaction**:
Since the reaction involves the reduction of AuCl₃ (where gold is reduced) and the oxidation of SnCl₂ (where tin is oxidized), this process is classified as a **reduction reaction**. The substance that causes the reduction (SnCl₂) is acting as a reducing agent.

5. **Conclusion**:
The method described in the reaction is an example of the **reduction method** for preparing colloidal solutions of metals.

### Final Answer:
The above method is an example of the **reduction method**.

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