You are correct, the correct answer is A) Slip the towline and allow it to run off the drum. This is the appropriate action because if the towed vessel is sinking in deep water, the safest course of action is to quickly release the towing hawser to avoid the towing vessel also being pulled under. Attempting to sever the towline may be difficult or unsuccessful, so slipping the line is the quickest way to disconnect from the sinking vessel. The other options are not recommended - abandoning the towing vessel (B) would unnecessarily endanger the crew, calling for emergency assistance (C) may not be possible in time, and securing watertight openings (D) would not prevent the towing vessel from being pulled under.