The correct answer is D. Commutation.
Armature reaction is the magnetic field produced by the current flowing through the armature windings. This field opposes the field produced by the field windings, which causes the terminal voltage of the generator to drop.
Armature resistance drop is the voltage drop across the armature windings due to the current flowing through them. This drop is proportional to the current and the resistance of the windings.
Field weakening due to armature reaction and armature resistance drop is the decrease in the field current due to the armature reaction and armature resistance drop. This decrease in field current causes the terminal voltage of the generator to drop.
Commutation is the process of transferring the current from one commutator segment to the next without sparking. This is done by ensuring that the brushes are always in contact with the commutator segments that are carrying current.
Commutation does not cause the terminal voltage of a D.C. shunt generator to drop.