Under the U.S. Aids to Navigation System, a yellow buoy is a __________.

Correct Answer: Option C — special purpose mark

Yellow buoys are designated as special purpose marks under 33 CFR 62.31. These marks are used to indicate areas such as anchorages, spoil grounds, or military exercise zones. They do not have lateral significance and are not intended to assist in channel navigation, but rather to identify specific water areas.