The correct answer is D) Squirrel-cage. The split-phase family of single-phase motors uses a squirrel-cage rotor. In a split-phase motor, the starting winding and the running winding are connected in parallel, with the starting winding being disconnected once the motor reaches a certain speed. The squirrel-cage rotor is well-suited for this type of motor design, as it is simple, robust, and can handle the starting torque requirements. The other options are incorrect because: A) Salient pole rotors are typically used in synchronous motors, not split-phase single-phase motors; B) Drum rotors are found in DC motors; and C) Wound-rotor designs are more common in polyphase induction motors, not single-phase split-phase motors.