What flag would a merchant ship fly to indicate it has dangerous cargo?

A merchant ship would fly a red flag known as the "Bravo" flag to indicate that it has dangerous cargo on board. This flag is part of the International Code of Signals and is universally recognized in maritime communication. The Bravo flag is used specifically to signify that the vessel is handling or transporting dangerous goods, which could include flammable materials, explosives, or toxic substances. This alert serves to inform other vessels and nearby personnel to exercise caution and maintain a safe distance for safety reasons.

Meanwhile, the other flags mentioned in the choices are indicative of different messages. The yellow flag typically signals caution, indicating that a ship may be quarantined or is in need of help but does not specifically denote dangerous cargo. The blue flag might suggest an alert but doesn't convey the specific danger associated with hazardous materials. The green flag is used to indicate safety or that a vessel is free of danger or unencumbered, which is the opposite of what is communicated by flying the Bravo flag. This distinct use of flags is crucial for maritime safety and effective communication on the water.