The correct answer is A. Neutralizing residual magnetism.
Flashing the field of a DC generator means applying a reverse DC voltage to the field windings for a short period of time. This reverse voltage causes the current in the field windings to flow in the opposite direction, which neutralizes the residual magnetism in the generator.
Residual magnetism is the magnetism that remains in the poles of a DC generator after it has been turned off. This residual magnetism can cause problems when the generator is turned on again, as it can cause the generator to produce a higher voltage than it is supposed to. Flashing the field neutralizes the residual magnetism, so that the generator produces the correct voltage when it is turned on.
Option B is incorrect because it is the opposite of what flashing the field does. Flashing the field neutralizes residual magnetism, not creates it.
Option C is incorrect because it is not related to the field of a DC generator. The magnetic losses of forces are a property of the magnetic circuit of a generator, and they are not affected by flashing the field.
Option D is incorrect because it is not the purpose of flashing the field. Flashing the field does not increase the flux density by adding extra turns of windings on poles.