The correct answer is: C. Is opposite to that of the main pole ahead.
Interpoles are auxiliary poles located between the main poles of a DC generator. They are used to reduce sparking at the brushes. The polarity of the interpoles is opposite to that of the main pole ahead. This helps to ensure that the current in the armature winding always flows in the same direction, which reduces sparking.
Option A is incorrect because the polarity of the interpoles is opposite to that of the main pole ahead, not the same.
Option B is incorrect because the polarity of the interpoles is opposite to that of the main pole ahead, not the same.
Option D is incorrect because the interpoles do play a part in generating e.m.f. They help to reduce sparking, which in turn improves the efficiency of the generator.