Which of the following reactions is an example of autoreduction ?

Options

A. `Fe_3O_4 + 4CO to 3Fe + 4CO_2`
B. `Cu_2O + C to 2Cu + CO`
C. `Cu^(2+) (aq) + Fe(s) to Cu(s) + Fe^(2+)(aq)`
D. `Cu_2O + 1//2 Cu_2S to 3Cu + 1//2 SO_2`

Explanation

To determine which reaction is an example of autoreduction (or self-reduction), we need to analyze the provided options and identify the reaction where a substance reduces itself while simultaneously oxidizing another component.

### Step-by-Step Solution:

1. **Understanding Autoreduction**:
- Autoreduction refers to a process where a substance undergoes reduction by itself while another component is oxidized. This typically occurs in reactions involving metal sulfides or oxides.

2. **Analyzing the Options**:
- We need to look at each option provided in the question and see if any of them fit the criteria for autoreduction.

3. **Identifying the Reaction**:
- In the reaction given in option D, copper sulfide (CuS) reacts with oxygen (O2). The initial reaction produces copper(I) oxide (Cu2O) and sulfur dioxide (SO2):
\[ 2CuS + 3O2 \rightarrow 2Cu2O + 2SO2 \]
- However, not all copper sulfide is converted into copper(I) oxide. The remaining copper sulfide can react with the copper(I) oxide formed:
\[ CuS + Cu2O \rightarrow 2Cu + SO2 \]
- In this step, the copper(I) oxide is reduced back to metallic copper, while the copper sulfide is oxidized to sulfur dioxide.

4. **Conclusion**:
- Since copper is being reduced from copper(I) oxide back to metallic copper while the sulfide is oxidized, this reaction exemplifies autoreduction. Therefore, option D is the correct answer.

### Final Answer:
**Option D is an example of autoreduction.**