A vessel at anchor, restricted in her ability to maneuver, will sound which fog signal?

A vessel at anchor that is restricted in her ability to maneuver sounds one prolonged blast followed by two short blasts every two minutes to signal its presence in restricted visibility, such as in fog. This specific signal is established in the International Regulations for Preventing Collisions at Sea (COLREGs) and is designed to help other vessels understand the operational status of the anchored vessel, thereby promoting safety on the water.

The prolonged blast indicates the vessel is present and can help other vessels avoid potential collisions. The subsequent two short blasts signify that the vessel's ability to maneuver is restricted, which is critical information for nearby vessels as it indicates that they must carefully navigate around it to avoid an accident. The timing of this signal (every two minutes) helps ensure that it is continuously communicated to other vessels, maintaining awareness in low-visibility conditions.

This specific sound signal is distinct and recognized within maritime rules, setting it apart from other options, which do not align with the requirements for a vessel at anchor under such circumstances.