The correct answer is D.
An equalizer bar is a copper bar that is connected between the armature windings of two DC generators that are running in parallel. It serves to reduce the combined effect of armature reaction of both the machines.
Armature reaction is the magnetic field produced by the current flowing through the armature windings of a DC generator. This field opposes the main field of the generator, which can cause a decrease in the generated voltage. The equalizer bar helps to reduce this effect by providing a path for the armature current to flow that is parallel to the main field. This reduces the magnetic field produced by the armature current, which in turn reduces the effect of armature reaction.
As a result, the equalizer bar helps to ensure that the two generators will pass approximately equal currents to the load. This is important because it ensures that the generators will share the load equally and that neither generator will be overloaded.
The other options are incorrect.
Option A is incorrect because the equalizer bar does not increase the series flux. The series flux is the magnetic field produced by the current flowing through the field windings of the generator. The equalizer bar does not affect the field windings, so it does not affect the series flux.
Option B is incorrect because the equalizer bar does not increase the generated e.m.f. The generated e.m.f. is the voltage produced by the generator. The equalizer bar does not affect the magnetic field produced by the main field, so it does not affect the generated e.m.f.
Option C is incorrect because the equalizer bar does not reduce the combined effect of armature reaction of both the machines. The equalizer bar actually helps to reduce the effect of armature reaction by providing a path for the armature current to flow that is parallel to the main field.