The correct answer is B) Excessive vibration of the vessel. In a turbo-electric drive system, the propulsion motor must remain in synchronization with the propulsion generator. If the motor drops out of synchronization, it can cause excessive vibration in the vessel, as the motor is no longer properly coupled to the propulsion shaft. This vibration is a clear indication that the motor has lost synchronization with the generator. The other answer choices are incorrect because they do not directly indicate a loss of synchronization. Closed contact in the field circuits, overheated crosstie busses, and tripped main motor interlocks may be symptoms of other issues, but they do not necessarily signify that the propulsion motor has dropped out of synchronization with the generator.