The correct answer is D: The parallel-connected generator which has suffered the loss of field excitation must be immediately disconnected from the bus by tripping its circuit breaker. When a generator operating in parallel with others loses its field excitation, it can no longer maintain proper voltage and frequency, and will begin to act as a motor, drawing power from the bus and potentially causing damage to the generator or the overall electrical system. To prevent this, the proper action is to immediately disconnect the affected generator from the bus by tripping its circuit breaker, isolating it from the parallel-connected generators. The other options are incorrect because: A) Over-exciting the other generators will not compensate for the loss of field on the affected generator; B) Stopping the prime mover will not address the underlying issue of the generator acting as a motor; and C) Reducing the prime mover to idle speed will not prevent the generator from drawing power from the bus.