A current perpendicular to a vessel's track has the greatest effect on the vessel's course made good __________.

Correct Answer: Option C — at low vessel speeds

Vector analysis dictates that the set and drift effect on a vessel's track is inversely proportional to speed. At lower speeds, the vessel spends more time within the current's influence, allowing the cross-track component to displace the vessel further from the intended track line, as explained in Bowditch.