The correct answer is: C. So that two similar machines will pass approximately equal currents to the load.
An equalizer bar is a copper bar that is connected between the two field coils of a DC series generator. It is used to equalize the current in the two field coils, which ensures that the two machines will pass approximately equal currents to the load.
Option A is incorrect because the speed of a DC series generator is determined by the voltage applied to the field coil. The equalizer bar does not affect the voltage applied to the field coil.
Option B is incorrect because the series flux is determined by the current in the field coil. The equalizer bar does not affect the current in the field coil.
Option D is incorrect because the equalizer bar does not reduce the combined effect of armature reaction of both machines. The armature reaction is caused by the magnetic field produced by the current in the armature winding. The equalizer bar does not affect the current in the armature winding.