The correct answer is A) yaw. The biggest problem encountered while towing a single tow astern is typically yaw, which is the tendency of the tow to swing from side to side. This is due to the forces acting on the tow, such as wind, current, and the motion of the towing vessel. Yaw can make the tow difficult to control and can lead to instability in the towing arrangement. The other answer choices are less common issues. B) Swamping of the tow is not a typical problem, as the tow should be properly secured and have adequate freeboard. C) The tow tending to dive is also not a common issue, as the tow should be trimmed and ballasted appropriately. D) The catenary dragging on the bottom is a concern, but it is usually secondary to the problem of yaw.