The correct answer is A. Silicon steel.
Armature core is the part of a DC generator that rotates and converts mechanical energy into electrical energy. It is made of silicon steel because it has high permeability and low hysteresis loss. This means that it can easily be magnetized and demagnetized, and it does not lose much energy when it is magnetized and demagnetized.
Copper is a good conductor of electricity, but it is not a good magnetic material. This means that it would not be a good choice for the armature core of a DC generator.
Nonferrous materials are materials that do not contain iron. They are not good magnetic materials, so they would not be a good choice for the armature core of a DC generator.
Cast-iron is a ferrous material, but it is not a good magnetic material. It is also a heavy material, which would make the DC generator more difficult to operate.