The correct answer is A) 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 loses its field excitation while operating in parallel with other generators, it will start to draw leading reactive power from the bus, which can lead to system instability and potential damage to the generator. The immediate solution is to disconnect the affected generator from the bus to prevent it from destabilizing the power system. The other options are incorrect because over-exciting the remaining generators, stopping the prime mover, or reducing it to idle speed will not address the root issue of the loss of field excitation in the affected generator.