The correct answer is B) armature current. The torque produced by a DC motor armature is the multiplication product of the density of the magnetic flux (which is dependent on the field current) and the armature current. This is because the torque is proportional to the interaction between the magnetic field and the current-carrying conductors in the armature. The other options are incorrect because: A) armature voltage drop is not a factor in torque production, C) armature conductance is related to resistance and not directly to torque, and D) armature resistance is also not a direct factor in torque production.