Explanation: For vessels of 500 gross tons and upwards, Global Maritime Distress and Safety System (GMDSS) regulations, specifically SOLAS Chapter IV, require the carriage of two radar transponders (SARTs) or AIS-SARTs. These transponders are crucial for assisting search and rescue operations by indicating the location of survival craft to search aircraft and ships equipped with radar. For smaller vessels (300-500 GT), only one SART is required. Therefore, for vessels over 500 gross tons, the requirement for a *second* radar transponder is the additional carriage requirement. Option A is incorrect because GMDSS specifies equipment for lifeboats, not a universal requirement for "two radio equipped lifeboats aft." Option C is incorrect as the requirement is for three portable GMDSS VHF radios for vessels over 500 gross tons, not "four additional." Option D is too broad and not a specific equipment carriage requirement for survival craft; their primary communication is for distress and on-scene coordination.