The correct answer is: Armature copper loss.
Hysteresis loss is the energy lost in the magnetic material of the generator due to the reversal of the magnetic field. Field copper loss is the energy lost in the resistance of the field windings. Armature copper loss is the energy lost in the resistance of the armature windings. Eddy current loss is the energy lost due to the eddy currents that are induced in the armature windings by the changing magnetic field.
At rated load, the armature current is the highest. Therefore, the armature copper loss is the highest.
Hysteresis loss is proportional to the square of the flux density. The flux density is not very high at rated load, so hysteresis loss is not very high.
Field copper loss is proportional to the square of the field current. The field current is not very high at rated load, so field copper loss is not very high.
Eddy current loss is proportional to the square of the frequency and the square of the ampere-turns. The frequency is not very high and the ampere-turns are not very high at rated load, so eddy current loss is not very high.