The correct answer is D) application of a three-phase voltage supply to the stator windings. In a three-phase, squirrel-cage type, induction motor, the primary rotating magnetic field is established by the application of a three-phase voltage supply to the stator windings. The three-phase voltage creates a rotating magnetic field in the stator, which then induces a current in the rotor's squirrel-cage bars. The interaction between the stator's rotating magnetic field and the rotor's induced current produces the torque that causes the rotor to rotate. The other options are incorrect because they do not accurately describe the mechanism for establishing the primary rotating magnetic field in a three-phase, squirrel-cage type, induction motor. The current induced in the rotor windings (option A) is a result of the rotating magnetic field, not the cause of it. The laminated steel core and aluminum conductors in the rotor (option B) are the physical components that make up the rotor, but they do not directly generate the primary rotating magnetic field. The interaction of the magnetic field caused by the induced current in the squirrel-cage bars with the magnetic field of the stator (option C) is a consequence of the rotating magnetic field, not the cause of it.