What type of buoy marks an obstruction?

The type of buoy that marks an obstruction is the yellow buoy. Yellow buoys are specifically used to indicate hazards such as submerged rocks, wrecks, or other dangers that are not navigable. They serve as a cautionary signal for mariners, warning them to proceed with care in the area.

In contrast, red and green buoys are typically used to mark safe navigable channels and help vessels keep track of their position in relation to the channel they are navigating. White buoys usually indicate non-navigational information or specific activities, such as a swimming area or other regulations, rather than marking hazards. Therefore, the unique role of yellow buoys as markers for obstructions makes them the correct choice in this context.