The correct answer is D) Transmit Distress call by MF/HF, VHF or INMARSAT. In a distress situation where immediate help is needed but the vessel is not sinking nor needs to be abandoned, the most appropriate action for a GMDSS Radio Operator is to transmit a distress call using the available communication methods, such as MF/HF, VHF, or INMARSAT. This is because the GMDSS (Global Maritime Distress and Safety System) regulations require the use of these communication methods to alert the Rescue Coordination Center (RCC) and request immediate assistance. The other options are incorrect because: A) Transmitting a radiotelegraph automatic alarm signal is not the most appropriate action in this scenario, as it may not effectively alert the RCC; B) Notifying the RCC through VHF FM on Channel 13 alone may not be sufficient to request immediate assistance; and C) Switching off the EPIRB (Emergency Position-Indicating Radio Beacon) and SART (Search and Rescue Transponder) manually would not be the appropriate action in this distress situation.