A vessel sounding a fog signal of one short, one prolonged, and one short blast is indicating that the vessel is __________.

When a vessel sounds a fog signal consisting of one short blast, one prolonged blast, and one short blast, it is signaling that the vessel is at anchor. This specific sequence of signals is part of the rules established for vessels navigating in reduced visibility conditions, such as fog.

Under the International Regulations for Preventing Collisions at Sea (COLREGs), vessels must use defined sound signals to communicate their status and intentions to other vessels when visibility is impaired. The sound of one short blast followed by one prolonged blast and another short blast distinctly identifies that the vessel is stationary at anchor, thereby alerting nearby vessels of its presence and ensuring safe navigation around it.

Other options mention different vessel statuses: "underway" would involve a different sound signal indicating that the vessel is moving and actively navigating; "restricted in ability to maneuver" would require a different series of signals to indicate that the vessel is unable to change course or speed due to operational limitations; and "maneuvering" would also necessitate unique sound signaling to convey its intentions. Thus, recognizing and interpreting these sound signals correctly is crucial for maritime safety and navigation.