What signal is used to indicate doubt that sufficient action is being taken by another vessel to avoid collision?

Answer: C — Five short and rapid blasts of the whistle

The signal indicating doubt that sufficient action is being taken by another vessel to avoid a collision is five short and rapid blasts of the whistle. This specific sound signal is universally recognized as a warning or a call for clarification when there is uncertainty about a vessel's intentions or actions. When one vessel believes that the other is not maneuvering effectively to prevent a potential collision, using five short blasts conveys urgency and serves as a way to prompt immediate awareness and caution from the other vessel. This type of signaling is particularly important in busy waterways or when visibility is impaired, ensuring that all operators are alerted to possible danger. Other signals like one long blast or two short blasts have different meanings and are used under other circumstances, such as indicating the presence of a vessel or signaling to inform another vessel of one's own maneuvering intentions. Three long blasts, on the other hand, is typically used to signify that a vessel is operating in a specific manner, such as engaging in a restricted maneuver.