By what common means is the speed of the AC propulsion motor on a diesel- electric propulsion ship controlled?

Correct Answer: Option A — by varying the input frequency of the voltage to the motor

Diesel-electric propulsion systems typically employ frequency-controlled AC drives. By varying the input frequency to the stator windings via a power electronic converter, the synchronous speed of the motor is adjusted. This method provides efficient, stepless speed control across the entire operating range, as outlined in modern marine electrical engineering standards.