Which of the following lights must a vessel exert when at anchor?

When a vessel is at anchor, it is required to display specific lights to signal its presence to other vessels. According to the International Regulations for Preventing Collisions at Sea (COLREGs), specifically Rule 30, a vessel at anchor must exhibit an all-round white light or one ball-shaped shape if it's a sailing vessel. The requirement for displaying these lights is crucial for ensuring visibility and safety in navigable waters, especially at night or in reduced visibility conditions.

The option that indicates the lights prescribed in Rule 30 correctly encapsulates these requirements, as it ensures that other vessels can see the anchored vessel and reduce the risk of collisions. Other options, such as only displaying an all-round white light, not showing any lights at all, or exhibiting the same lights as when the vessel is underway, do not encompass the complete regulations set forth for vessels at anchor. Therefore, Rule 30 provides the definitive guidance for this situation, making it the correct choice.