What would a vessel towing a barge show at the stern of the tow vessel?

Answer: A — towing light above the stern light.

A vessel towing a barge is required to display a towing light above the stern light at the stern of the tow vessel. This is crucial for ensuring that other vessels can clearly identify the nature of the tow and the special navigation situation. The towing light serves as an important signal that indicates the vessel is both a towing vessel and that it has a tow in the water, which may extend beyond its normal length. By having this towing light positioned above the stern light, it provides an additional visual cue, especially at night or during reduced visibility conditions, helping to prevent collisions and improve situational awareness for nearby vessels. The other options do not meet the requirements set by maritime navigation rules. While a sign indicating the vessel's length may be useful information, it's not a regulatory light requirement. Similarly, stating that no additional lights are needed or that only a red light should be shown does not comply with the standards for a towing vessel, which must include a towing light to convey its specific status on the water.