The correct answer is A. Zero.
A DC generator is a device that converts mechanical energy into electrical energy. It does this by using a rotating magnetic field to induce an electric current in a coil of wire. The amount of voltage produced by a DC generator depends on the strength of the magnetic field, the speed of rotation, and the number of turns in the coil.
When a DC generator is run at full speed without any excitation, there is no magnetic field. This means that there will be no induced current in the coil, and the open circuit voltage will be zero.
Option B is incorrect because a DC generator will produce a voltage even if it is not fully excited. However, the voltage will be much lower than the rated voltage of the generator.
Option C is incorrect because a DC generator will produce a voltage even if it is not fully excited. However, the voltage will be much lower than the rated voltage of the generator.
Option D is incorrect because a DC generator will not produce any voltage if it is not fully excited.