What flag must divers display to warn other boaters?

The correct choice is the red and white diver's flag, which is commonly recognized as a warning signal to alert other boaters that divers are in the water. This flag, often referred to as the diver down flag, features a red background with a white diagonal stripe running from top to bottom, left to right. When displayed, it indicates that there are divers below, and boaters must take caution, avoiding that area and keeping a safe distance to prevent accidents.

This flag serves an important purpose in ensuring the safety of divers, as it communicates their presence to nearby boats, particularly in busy waterways. The use of this specific flag is standardized and recognized internationally, making it a vital tool for promoting safe boating practices in conjunction with diving activities. Other flags mentioned do not represent diver safety and might be associated with different meanings or activities, thus rendering them unsuitable for this situation.