Explanation:
The correct answer is option 'D' (None of the mentioned). Let's understand why.

Open circuit characteristic:
- The open circuit characteristic of a DC generator is a graph that represents the relationship between the generated emf (voltage) and the field current at no-load condition.
- This characteristic is obtained by disconnecting the load from the generator and measuring the generated voltage at different field current values.
- It helps in determining the voltage regulation of the generator and is important for proper operation and control of the generator.

Magnetization characteristic:
- The magnetization characteristic of a DC generator is a graph that represents the relationship between the field current and the field flux produced by the generator.
- It shows how the field current affects the magnetic field strength of the generator.
- This characteristic is important for determining the field current required to produce the desired magnetic field strength for proper generator operation.

Conducted on the unloaded machine:
- The no-load characteristics of a DC generator are conducted on the unloaded machine, meaning there is no load connected to the generator.
- This allows for accurate measurements of the generated voltage and the field current at different operating points.
- The no-load characteristics provide important information about the generator's performance and help in determining its operating parameters.

Why option 'D' is the correct answer?
- None of the mentioned options are inappropriate for the no-load characteristics of a DC generator.
- Options 'A' and 'B' accurately describe the open circuit characteristic and magnetization characteristic, which are important aspects of the no-load characteristics.
- Option 'C' correctly states that the no-load characteristics are conducted on the unloaded machine, which is necessary to obtain accurate measurements.
- Therefore, option 'D' (None of the mentioned) is incorrect as all the given options are appropriate and related to the no-load characteristics of a DC generator.

Conclusion:
In summary, the most inappropriate option out of the given choices for the no-load characteristics of a DC generator is option 'D' (None of the mentioned). All the other options accurately describe different aspects of the no-load characteristics, including the open circuit characteristic, magnetization characteristic, and the fact that it is conducted on the unloaded machine.