The correct answer is C) port field controller. If the port propulsion motor field excitation circuit experiences a failure, the port field controller would need to fail in order for the standby excitation transformer and standby field controller to be used to resume normal operation. The port field controller is the component responsible for regulating the field excitation to the port propulsion motor. A failure in this component would prevent the normal field excitation from being provided, thereby requiring the use of the standby system. The other options are incorrect because they do not directly control the field excitation to the motor. The port rotary transformer, port rotating rectifier, and port motor field winding are all part of the overall field excitation circuit, but a failure in these components would not necessarily allow the standby system to be utilized.