The standard fixed point for temperature measurements in use today is

Options

A. melting point of ice at one atmospheric pressure
B. of pure ice and pure water in equilibrium at one atmosphere pressure
C. at which ice, water and watervapour coexist
D. none of the above

Answer

To solve the question regarding the standard fixed point for temperature measurements, we will analyze the options provided and determine the correct answer step by step.

### Step-by-Step Solution:

1. **Understanding the Concept of Fixed Points**:
- Fixed points in temperature measurement are specific conditions under which a substance can be used to define a temperature scale. These points are typically based on phase changes of a substance.

2. **Reviewing the Options**:
- **Option A**: Melting point of ice at one atmospheric pressure.
- **Option B**: Pure ice and pure water in equilibrium at one atmospheric pressure.
- **Option C**: The point at which ice, water, and water vapor coexist.
- **Option D**: None of the above.

3. **Analyzing Each Option**:
- **Option A**: The melting point of ice is indeed a fixed point, but it is not the standard fixed point currently in use.
- **Option B**: This describes the equilibrium between ice and water, which is also a fixed point but not the standard one.
- **Option C**: The triple point of water is defined as the condition where ice (solid), water (liquid), and water vapor (gas) coexist in equilibrium. This is a specific and well-defined point used in thermometry.
- **Option D**: This option suggests that none of the previous options are correct, which is not true since option C is correct.

4. **Conclusion**:
- The correct answer is **Option C**: The point at which ice, water, and water vapor coexist, known as the triple point of water. This is the standard fixed point for temperature measurements in use today.

### Final Answer:
The standard fixed point for temperature measurements in use today is the triple point of water (Option C).