What must vessels do to indicate they are at anchor, as per Rule 27?

Answer: B — Display lights and shapes

Vessels must display specific lights and shapes to indicate that they are at anchor, as outlined in Rule 27 of the COLREGs. When at anchor, a vessel is required to show one white anchor light at the stern and, if the vessel is over 50 meters in length, it also must display a second white light at the bow. Additionally, vessels are required to exhibit a ball shape during the day. This combination of lights and shapes serves to inform other vessels of their status, helping to prevent collisions and ensuring safe navigation. The other options mention various practices that are important for safe operation at sea but do not align with the specific requirements for indicating a vessel at anchor as stated in the COLREGs. For example, raising a flag is not a requirement for indicating an anchored status, and while sounding a horn may be used for other navigational signals or in instances of reduced visibility, it does not specifically communicate that a vessel is at anchor. Maintaining a watch on deck is a best practice for safe navigation but does not serve as an indication of being at anchor.