Which buoy is often referred to as a "can" buoy?

The choice of an unlighted can buoy is correct because it specifically refers to a type of navigational aid that has a cylindrical shape resembling a can. This design feature makes it easily identifiable in various marine environments. Can buoys are part of the lateral marking system used to indicate the edges of safe navigation channels. They usually mark the port side of the channel when proceeding from open sea toward the harbor and are typically colored green.

In the context of the other options, a lighted buoy can also be a can buoy, but the question specifically asks for the "can" buoy without any indication of it being lighted. A nun buoy, on the other hand, is a different type of lateral buoy that has a conical shape and indicates the starboard side of a channel. Lastly, a square buoy is not a standard term in the navigation aid system and generally does not refer to recognized buoy types. Thus, the unlighted can buoy is the correct designation for the specific buoy referenced.