In restricted visibility, a vessel restricted in her ability to maneuver, at anchor, would sound a fog signal of __________.

In restricted visibility, a vessel that is restricted in her ability to maneuver and is at anchor is required to signal her presence using specific sound signals to ensure safety on the water. The correct sound for a vessel at anchor in restricted visibility is one prolonged blast followed by two short blasts, and this should be made at intervals of two minutes.

The prolonged blast indicates the presence of the vessel and warns other mariners of her position, while the two short blasts serve as a further signal to clarify that the vessel is at anchor and restricted in her ability to maneuver. This combination helps other vessels nearby understand the state of the anchored vessel, allowing for safer navigation in limited visibility conditions.

The other answer choices do not conform to the established rules for sound signals in restricted visibility for a vessel at anchor, which further emphasizes the importance of using the correct signal to ensure maritime safety. Understanding these signals is crucial for all mariners to avoid potential collisions and maintain situational awareness in fog or other conditions where visibility is reduced.