The correct answer is C. May be greater or less than 250 V.
The open circuit voltage of a compound generator is the voltage measured across the terminals of the generator when there is no load connected. The terminal voltage of a generator is the voltage measured across the terminals of the generator when there is a load connected.
The terminal voltage of a generator is always less than the open circuit voltage because of the internal resistance of the generator. The internal resistance of the generator causes a voltage drop across the generator when there is a load connected.
The amount of voltage drop across the generator depends on the amount of current flowing through the generator. The amount of current flowing through the generator depends on the amount of load connected to the generator.
If the load connected to the generator is small, the voltage drop across the generator will be small and the terminal voltage will be close to the open circuit voltage. If the load connected to the generator is large, the voltage drop across the generator will be large and the terminal voltage will be much less than the open circuit voltage.
Therefore, the terminal voltage of a compound generator may be greater or less than 250 V, depending on the amount of load connected to the generator.