The correct answer is: A stronger commutating field is needed at low speed than at high speed.
A commutating field is a magnetic field that is used to switch the current in the armature windings of a DC motor. The strength of the commutating field needs to be greater at low speeds than at high speeds in order to ensure that the current is switched smoothly and efficiently. This is because the armature windings have a higher inductance at low speeds, which means that they resist changes in current. A stronger commutating field is needed to overcome this resistance and ensure that the current is switched smoothly.
Option B is incorrect because a weaker commutating field would not be able to overcome the resistance of the armature windings and would result in poor commutation. Option C is incorrect because the strength of the commutating field does need to change with speed. Option D is incorrect because one of the options is correct.