The correct answer is D) maintain a strict watch on the barge from the towing vessel. When towing an unmanned tank barge, the regulations require the towing vessel to maintain a strict watch on the barge at all times. This is to ensure the safety of the tow and to monitor the barge for any issues that may arise during the voyage. The other options are incorrect because they do not address the primary requirement of maintaining a proper watch on the barge.