The correct answer is D) five years. The approval period for a shipboard Oil Pollution Emergency Plan (SOPEP) is five years, as per the requirements set forth by the United States Coast Guard and the International Convention for the Prevention of Pollution from Ships (MARPOL). The SOPEP is a critical document that outlines the procedures and actions to be taken in the event of an oil spill or other pollution incident on board a vessel, and it must be reviewed and reapproved every five years to ensure it remains up-to-date and effective. The other options, A) two years, B) three years, and C) four years, are incorrect as they do not align with the established five-year approval period for a SOPEP.