The correct answer is: A. Series generator
A series generator is a type of DC generator in which the field winding is connected in series with the armature winding. This means that the current flowing through the field winding is the same as the current flowing through the armature winding. When a series generator is running on no-load, there is no current flowing through the armature winding. This means that there is no magnetic field produced by the field winding, and the terminal voltage of the generator is negligible.
A shunt generator is a type of DC generator in which the field winding is connected in parallel with the armature winding. This means that the current flowing through the field winding is independent of the current flowing through the armature winding. When a shunt generator is running on no-load, there is a small amount of current flowing through the armature winding. This small amount of current produces a magnetic field, which causes the terminal voltage of the generator to be slightly higher than zero.
A compound generator is a type of DC generator that has both a series field winding and a shunt field winding. The series field winding is connected in series with the armature winding, and the shunt field winding is connected in parallel with the armature winding. When a compound generator is running on no-load, the current flowing through the series field winding is much higher than the current flowing through the shunt field winding. This means that the magnetic field produced by the series field winding is much stronger than the magnetic field produced by the shunt field winding. The strong magnetic field produced by the series field winding causes the terminal voltage of the generator to be much higher than zero.
A separately excited generator is a type of DC generator in which the field winding is connected to a separate DC source. This means that the current flowing through the field winding is independent of the current flowing through the armature winding. When a separately excited generator is running on no-load, the current flowing through the field winding is controlled by the DC source. This means that the magnetic field produced by the field winding can be controlled, and the terminal voltage of the generator can be set to any desired value.