Ungrounded systems must be marked with a caution sign indicating what information?

The correct choice highlights the critical safety information that must be communicated when working with ungrounded systems. Ungrounded systems can present unique hazards, particularly because they can maintain a live conductor even when faults occur that would typically cause a grounded system to trip or shut down. By indicating "Caution: Ungrounded System Operating Volts Between Conductors," the sign provides essential information regarding the operational voltage levels, making clear the potential shock hazard. This awareness is crucial for personnel who may be working near or with the system, as it signals that special care must be taken due to the nature of ungrounded systems. Proper marking ensures safety practices can be implemented, reducing the risk of accidents or injuries associated with electrical work in these environments.