What is required of vessels at anchor according to Rule 27?

Answer: B — They must display an appropriate light and shapes for visibility

Vessels at anchor are required to display an appropriate light and shapes for visibility as stipulated in Rule 27 of the COLREGs. This requirement is crucial for ensuring that vessels can be seen by other vessels in the vicinity, especially during periods of reduced visibility, such as at night or in foggy conditions. The 'appropriate light' typically refers to a white light that is visible all around the horizon, while 'shapes' refers to specific visual signals, such as a black ball, that indicate a vessel is at anchor. This helps other mariners recognize that the vessel is stationary and not navigating, thus preventing potential collisions. While identification, sound signals, and crew presence might be beneficial in certain situations, the primary focus under Rule 27 is on the visibility through lights and shapes to enhance safety in navigation.