117. Which of the following terms is defined as a back-up power source that provides power to radio installations for the purpose of conducting Distress and Safety communications when the vessel's main and emergency generators cannot?

The correct answer is Reserve Source of Energy. This term, as defined in SOLAS Chapter IV, specifies a dedicated power source for GMDSS radio equipment that operates independently of the ship's main and emergency generators. Other options are inaccurate or use non-standard terminology.