Which light configuration indicates that a vessel is aground?

Answer: A — Red on red all-around lights

The configuration of red on red all-around lights indicates that a vessel is aground. According to the COLREGs, when a vessel is not under command, which includes situations like being aground, it must display two all-around red lights in a vertical line. This signals to other vessels that the vessel is unable to maneuver, providing crucial information for navigational safety. The other configurations do not convey this specific situation. Green and white lights typically pertain to other conditions, such as the vessel being underway and engaged in specific activities. A single white light could signify other statuses but does not communicate the grounding status. Similarly, blue lights on the starboard side are used for different contexts, such as fishing and not for indicating that a vessel is aground. Thus, the red on red configuration is the clear, standardized signal for a vessel that is not maneuverable due to being aground.