A 200-meter vessel is aground in fog. Which signal is optional?

Answer: A — whistle signal.

In situations where a vessel is aground, especially in conditions of reduced visibility such as fog, the Rules of the Road specify certain sound signals to indicate the vessel's status to others in the vicinity. A vessel aground must sound a signal every two minutes, which can be fulfilled through several methods. A whistle signal is not a mandatory requirement in this scenario, as the vessel's aground status means noise is primarily communicated through sound signals that most effectively convey the presence of the vessel. Nevertheless, a vessel may use a whistle if equipped, but it is not required, thus making it optional. In contrast, bell signals, gun signals, and flashing light signals are stipulated as necessary for a vessel aground. The bell signal is specifically required for vessels over 100 meters in length when aground, and all vessels must have the capacity to use the appropriate signals to wake alert nearby vessels of their location and status. This ensures safety and proper communication in potentially hazardous conditions.