The correct answer is D) being in step. When a towing vessel is towing another vessel, it is important for the towing vessel to be "in step" with the tow. This means that the towing vessel should be on the crest of a wave at the same time as its tow, and in the trough at the same time. This ensures that the tow is not subjected to excessive strain or forces, which could potentially damage the tow or the towing vessel. Being "in step" is the term used to describe this synchronized movement between the towing vessel and its tow. The other options are incorrect because they do not accurately describe the relationship between the towing vessel and its tow. Catenary length refers to the curved shape of the towing line, Williamson's Tow is a specific towing technique, and tow strain refers to the forces acting on the tow, but none of these terms specifically capture the concept of being "in step" with the tow.