The correct answer is: A. 2 to 3 percent.
Residual magnetism is the magnetism that remains in a ferromagnetic material after it has been removed from a magnetic field. It is caused by the alignment of the magnetic domains in the material. The amount of residual magnetism depends on the material and the strength of the magnetic field.
In D.C. machines, the residual magnetism is of the order of 2 to 3 percent. This means that after the machine has been turned off, there will still be a small amount of magnetism left in the stator and rotor. This residual magnetism can cause problems if the machine is started again without first being demagnetized.
The other options are incorrect because they are too high. Residual magnetism is usually much less than 10 percent.