The correct answer is: C. Irrespective of the value of load current.
A series generator can self-excite irrespective of the value of load current. This is because the field current is directly proportional to the armature current. When the generator is first started, the field current is zero and the output voltage is also zero. However, as the armature current increases, the field current also increases, which causes the output voltage to increase. This process continues until the output voltage reaches a steady-state value.
The options A and B are incorrect because they state that the series generator can only self-excite if the load current is zero or not zero, respectively. This is not true, as the series generator can self-excite irrespective of the value of load current.
Option D is incorrect because it states that none of the above is true. However, option C is true, so this option must be incorrect.