The `pH` indicators are

Options

A. Salts of strong acids and strong bases
B. Salts of weak acids and weak bases
C. Either weak acids or weak bases
D. Either strong acids or strong base

Explanation

To answer the question about pH indicators, we will analyze the options provided and determine which one correctly describes pH indicators.

### Step-by-Step Solution:

1. **Understanding pH Indicators**:
- pH indicators are substances that change color in response to changes in pH. They are used to determine whether a solution is acidic, basic, or neutral.

2. **Identifying Types of Indicators**:
- Indicators can be classified based on their chemical nature. They are typically either weak acids or weak bases. This is because weak acids and weak bases can undergo dissociation in a solution, leading to a change in color at specific pH levels.

3. **Analyzing the Options**:
- **Option 1**: Salts of strong acids and strong bases - This is incorrect. Salts do not change color based on pH.
- **Option 2**: Salts of weak acids and weak bases - This is also incorrect. While these salts can have buffering properties, they do not serve as indicators.
- **Option 3**: Either weak acids or weak bases - This is correct. Many common pH indicators, such as phenolphthalein and litmus, are weak acids or weak bases.
- **Option 4**: Either strong acids or strong bases - This is incorrect. Strong acids and bases do not function as indicators since they do not change color based on pH.

4. **Conclusion**:
- The correct answer is **Option 3: Either weak acids or weak bases**. This aligns with the definition and function of pH indicators.

### Final Answer:
**Option 3: Either weak acids or weak bases.**