The correct answer is C) reduce eddy current losses. The armature cores in a DC generator are made of laminated steel sheets to reduce eddy current losses. Eddy currents are induced electrical currents that circulate within the core material, causing energy losses and heating. By using laminated steel sheets instead of a solid core, the paths for these eddy currents are broken, significantly reducing the energy losses and improving the efficiency of the generator. The other options are incorrect because: A) easy assembly is not the primary reason for using laminated cores, B) the hysteresis effect is not the main concern, and D) fitting the curvature of the frame is not the driving factor behind the laminated core design.