According to Rule 26, what must vessels aground display?

Answer: B — Specific lights and shapes

Under Rule 26 of the COLREGs, vessels that are aground are required to display specific lights and shapes to indicate their situation to other vessels. This is crucial for maritime safety, as it alerts nearby boats that the aground vessel is not navigating normally and might pose an obstruction. The rule specifies that a vessel aground must display two all-round red lights in a vertical line as well as a black ball shape during the day. These signals are universally recognized maritime indicators, enabling other vessels to accurately assess the status of the aground craft, allowing for safe navigation in the vicinity. The other options mentioned may relate to safety but are not specific requirements outlined by Rule 26 for a vessel aground. For instance, emergency flares are typically used to signal distress rather than to indicate being aground specifically, while reflective markers and flags indicating distress have different purposes in maritime signaling and do not fulfill the requirements of Rule 26.