What is true regarding the day signals used by anchored vessels?

The correct choice highlights that day signals for anchored vessels are not strictly required. Under the International Regulations for Preventing Collisions at Sea (COLREGs), vessels at anchor are encouraged to display lights and shapes, but this requirement is not mandatory for all types of vessels. Specifically, while larger vessels and commercial ships may be subject to regulations that recommend these signals for safety and visibility, it is not a universal obligation. Therefore, smaller vessels or those that do not fall under specific regulatory frameworks regarding day signals have the discretion not to use them.

The other options suggest a requirement for day signals among all vessels, only large vessels, or only commercial vessels, which does not align with the flexibility provided under the regulations for different types of vessels. Vessels may choose to use day signals at anchor for better visibility and safety, but it is not mandated for all scenarios.