The correct answer is C) To provide sparkless commutation without having to shift the brushes. Interpoles are auxiliary poles installed in DC motors between the main poles. Their primary function is to create a magnetic field that opposes the armature reaction, which can cause sparking at the commutator. This allows for stable and sparkless commutation without the need to mechanically shift the position of the brushes as the motor speed changes. The other options are incorrect because: A) Interpoles do not strengthen the main field, B) Interpoles do not limit counter-electromotive force, and D) Interpoles do not limit the starting surge current. These are not the primary functions of interpoles in DC motors.