What do three all-around lights in a vertical line indicate?

Three all-around lights arranged in a vertical line signify that the vessel is engaged in diving operations. This configuration is specifically designated by the COLREGs (International Regulations for Preventing Collisions at Sea) to alert other vessels to the fact that there are divers in the water, and they should proceed with caution to avoid interfering with the diving activity.

The positioning of the lights ensures maximum visibility from all angles, making it clear to other vessels that they should stay clear of the area where diving is taking place. This is crucial for the safety of the divers and to prevent accidents in the vicinity.

While other choices might represent different scenarios that vessels could encounter, the vertical arrangement of three all-around lights is a definitive signal specifically reserved for diving operations, thereby confirming the correct response.