What type of buoy is an unlighted green buoy?

An unlighted green buoy is classified as a can buoy. Can buoys are cylindrical in shape and are typically painted green. They are used in lateral marking systems to indicate the port side of a channel when entering from the open sea or the service area of a harbor. This color and shape help mariners identify the correct navigational aids while navigating waterways.

The other options are not correct in this context. A nun buoy is an unlighted red buoy that denotes the starboard side of a channel. A safe-water buoy, distinguished by its red and white vertical stripes, provides navigational information but is not categorized as a lateral buoy like can buoys or nun buoys. Lastly, a nun marker, while similar in that it indicates a lateral marker, is actually a variation of the nun buoy and is not typically referred to separately as a distinct type under the same standards used to categorize can buoys. Thus, the can buoy designation accurately describes the unlighted green buoy mentioned in the question.