The correct answer is A) annually. The Oil Pollution Emergency Plan for a vessel is required to be reviewed and updated annually according to the Coast Guard regulations. This requirement is specified in 33 CFR 155.1030(c), which states that "The plan must be regularly reviewed and updated by the vessel owner or operator at least annually." The other answer choices are incorrect because they do not align with the Coast Guard's annual review requirement for the Oil Pollution Emergency Plan. Reviewing the plan less frequently, such as every two, four, or five years, would not meet the regulatory standard.