The correct answer is D) All of the above. Re-liquefaction plants on liquefied gas carriers perform all three of the listed functions: 1) They re-liquify cargo vapor generated during loading and return it to the cargo tanks, preventing venting of cargo and reducing cargo loss. 2) They can also be used to cool down the cargo tanks and piping systems prior to loading, ensuring the cargo system is at the proper temperature and pressure. 3) During transport, the re-liquefaction plant keeps the cargo at the required temperature and pressure to maintain the cargo in a liquid state within the design limits of the cargo system. The other options are incorrect because they do not encompass all the key functions of a re-liquefaction plant on a liquefied gas carrier.