The correct answer is D) five years. The periodic surveys to renew the load line assignment must be made at intervals not exceeding five years, as per the regulations governing load line certificates. Load lines are assigned to ships to indicate the maximum draft to which a vessel can be safely loaded, and these load line assignments must be periodically renewed through surveys to ensure the vessel's continued compliance with the applicable load line requirements. The other options are incorrect because 18 months and two years are too short for the required interval, while three years is shorter than the mandated five-year period for these periodic load line surveys.