The correct answer is D) Separately excited generator. The Ward-Leonard drive system uses a separately excited DC generator. In this configuration, the field winding of the generator is supplied with current from a separate source, typically a separate exciter, rather than being self-excited or permanently magnetized. This allows for precise control of the generator field and, consequently, the voltage output, which is essential for the proper operation of the Ward-Leonard drive system. The other options are incorrect because: A) a self-excited shunt wound generator uses the generator's own armature current to excite the field winding; B) a permanent magnet generator does not have a separate field winding; and C) a self-excited series wound generator uses the armature current to excite the field winding, which is not the case in the Ward-Leonard drive system.