The correct answer is D) five years. According to 33 CFR 155.1065(a), the approval period for a shipboard oil pollution emergency plan expires after five years. This regulation requires vessels to have an approved oil pollution emergency plan that must be resubmitted and approved every five years to ensure the plan is up-to-date and effective. The other options are incorrect because the approval period is not one year (A), two years (B), or four years (C) as stated in the question, but rather five years as per the regulatory requirement in 33 CFR 155.1065(a).