The correct answer is C.
The inter-pole flux should be sufficient to neutralize both the armature reaction flux as well as commutating e.m.f. induced in the coil.
The armature reaction flux is the flux produced by the current flowing through the armature windings. This flux interacts with the main field flux to produce a net flux that can cause the brushes to shift from their correct position. The commutating e.m.f. is the e.m.f. that is induced in the coil as it is being commutated. This e.m.f. can cause sparking at the brushes.
The inter-pole flux is produced by the current flowing through the inter-pole windings. This flux opposes the armature reaction flux and helps to keep the brushes in their correct position. It also helps to reduce sparking at the brushes.
If the inter-pole flux is not sufficient, the armature reaction flux will cause the brushes to shift from their correct position. This can cause sparking at the brushes and can also lead to a loss of power.
If the inter-pole flux is too strong, it can cause the coil to overheat. This can damage the coil and can also lead to a loss of power.
Therefore, the inter-pole flux should be sufficient to neutralize both the armature reaction flux as well as commutating e.m.f. induced in the coil.