Which words require the sounding of a danger signal?

The correct choice highlights the importance of specific words that indicate an imminent threat or a critical situation that warrants immediate attention and action. The words "doubt or danger" both encompass situations where a vessel may be at risk or is facing uncertainty, necessitating a danger signal.

When navigating, it is essential to communicate effectively about the safety and well-being of all onboard. The presence of "danger" clearly indicates that there is an immediate threat, and a danger signal must be used to alert others in the vicinity to the potential for a serious incident. In maritime operations, signaling danger enables vessels to take appropriate precautions, thereby enhancing safety and preventing accidents.

The other options do not carry the same level of urgency or imply a necessity for a danger signal. While "collision or emergency" might suggest serious situations, the requirement for a specific danger signal is more closely aligned with the certainty of "danger" itself. Caution and warning are advisory terms rather than clear signals that call for immediate action. Similarly, "stop or ready" relate to operational commands rather than alerts concerning safety risks.