What is required for a vessel less than 12 meters in length at anchor?

A vessel that is less than 12 meters in length is required to display an all-round white light when it is at anchor. This requirement is in accordance with international navigation rules, which aim to ensure visibility and safety on the water during nighttime or low visibility conditions. The all-round white light allows other vessels to see the anchored boat from all directions, significantly reducing the risk of collision.

Essentially, the use of the all-round white light helps other mariners identify the presence of an anchored vessel and ascertain that it is not underway. This is particularly important in busy waterways or during poor visibility situations, where distinguishing stationary vessels from moving ones can become challenging.

The other choices suggest alternative signals, which are not adequate or appropriate for a vessel at anchor. The red and white shape, for example, is generally not a standard requirement for vessels at anchor. The mention of only one black ball refers to a different signaling method used for indicating that a vessel is aground, which is also not applicable for a vessel merely at anchor. Lastly, any combination of white lights could lead to confusion or misinterpretation of the vessel's status, which is why the all-round white light is specifically mandated.