The correct answer is: C. 10 percent
Armature reaction is the effect of the armature current on the magnetic field produced by the field windings in a DC generator. The armature current creates a magnetic field that opposes the field produced by the field windings. This results in a reduction in the total mutual air gap flux in the generator.
The amount of reduction in the total mutual air gap flux depends on the magnitude of the armature current. The higher the armature current, the greater the reduction in the total mutual air gap flux.
In general, the reduction in the total mutual air gap flux due to armature reaction is approximately 10%. However, this value can vary depending on the design of the generator.
Option A is incorrect because it is too high. The reduction in the total mutual air gap flux due to armature reaction is typically much less than 40%.
Option B is incorrect because it is too low. The reduction in the total mutual air gap flux due to armature reaction is typically much more than 25%.
Option D is incorrect because it is too low. The reduction in the total mutual air gap flux due to armature reaction is typically much more than 5%.