The correct answer is: Armature.
The armature is the rotating part of a DC generator. It is made up of a number of coils of wire that are wound around a laminated iron core. When current flows through the coils, a magnetic field is created. This magnetic field interacts with the magnetic field of the stator (the stationary part of the generator) to produce an electric current.
The critical resistance of a DC generator is the resistance of the armature when the generator is operating at its maximum power output. This resistance is important because it determines the amount of current that flows through the armature. If the resistance is too high, the generator will overheat and may be damaged. If the resistance is too low, the generator will not be able to produce enough power.
The other options are incorrect because they do not refer to the resistance of the armature. The brushes are used to collect current from the rotating armature and transfer it to the stationary load. The field coils are used to create the magnetic field that interacts with the armature coils to produce an electric current. The load is the device that uses the electric current produced by the generator.