Which of the following generators will be preferred if they are required to be run in parallel?

When DC generators are required to be run in parallel, compound generators are preferred due to their better voltage regulation and stability in load sharing.

Step 1: Understanding Different Types of DC Generators
- Shunt Generators: They have poor voltage regulation under varying loads, which makes them less suitable for parallel operation.
- Series Generators: They cannot be used in parallel because their terminal voltage varies significantly with load changes.
- Compound Generators: These have both shunt and series windings, providing better voltage regulation and allowing them to share loads effectively in parallel operation.

Step 2: Why Compound Generators Are Preferred?
- They maintain a stable voltage when loads change.
- They ensure equal load sharing between generators.
- They prevent overloading of a single generator when connected in parallel.

Step 3: Analyzing the Given Options
- Option A: Shunt generators (Not preferred due to poor voltage regulation under varying loads.)
- Option B: Series generators (Not suitable as they do not work effectively in parallel operation.)
- Option C: Compound generators (Correct, as they provide better voltage stability and load sharing.)
- Option D: None of the above (Incorrect, as compound generators are the correct choice.)

Conclusion: For parallel operation, compound generators are preferred because of their good voltage regulation and stable load sharing.