Identify a useful change among the following chemical changes.

Options

A. blackening of silverware.
B. rusting of iron.
C. curdling of milk.
D. all the above.

Explanation

To identify a useful change among the given chemical changes, we need to analyze each option provided.

### Step-by-Step Solution:

1. **Identify the Changes**: We are given four options that represent different chemical changes. We need to evaluate each one to determine if it is useful.

2. **Option 1: Blackening of Silverware**
- **Chemical Reaction**: The reaction can be represented as:
\[ \text{Ag (solid)} + \text{H}_2\text{S (gas)} \rightarrow \text{Ag}_2\text{S (solid)} \]
- **Analysis**: The blackening of silverware occurs due to the formation of silver sulfide, which is black in color. This change is not useful as it tarnishes the silverware.

3. **Option 2: Rusting of Iron**
- **Chemical Reaction**: The rusting process can be represented as:
\[ \text{Fe (solid)} + \text{O}_2 + \text{H}_2\text{O} \rightarrow \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} \]
- **Analysis**: Rusting leads to the deterioration of iron, making it weaker and less useful. Thus, this change is also not useful.

4. **Option 3: Curdling of Milk**
- **Chemical Process**: Curdling of milk occurs due to the action of bacteria (like Lactobacillus) that ferment lactose, resulting in curd formation.
- **Analysis**: Curd is a valuable food product, rich in nutrients and widely consumed. Therefore, this change is useful.

5. **Option 4: All of These**
- **Analysis**: Since options 1 and 2 are not useful changes, this option cannot be correct.

### Conclusion:
After evaluating all the options, the only useful change among the provided chemical changes is **Option 3: Curdling of Milk**.

### Final Answer:
The correct answer is **Option 3: Curdling of Milk**.

---