The correct answer is B) back pressure trip. The back pressure trip is a safety device provided on a turbo generator that automatically closes the throttle when the cooling water to the condenser is insufficient. This is because a lack of cooling water can lead to an increase in the back pressure in the turbine, which can cause damage to the equipment. The back pressure trip is designed to detect this increase in back pressure and shut down the turbine to prevent further damage. The other options are incorrect because they refer to different types of safety devices used in other contexts. For example, an overspeed trip would be used to shut down the turbine if the rotational speed became too high, while a low pressure trip and a high temperature trip would be used to respond to other abnormal conditions in the system.