The correct answer is D) approach the lock under full control. The flashing amber light on the lock wall is a signal to the vessel operator to approach the lock slowly and under full control. This is to ensure the safe and controlled entry into the lock, which is a critical maneuver that requires careful handling of the vessel. Approaching the lock too quickly or without proper control could result in damage to the vessel, the lock, or even injury to the crew. The other options are incorrect because A) hanging off the tow on the lock wall is an unsafe and unauthorized action, B) entering the lock as quickly as possible would be reckless and dangerous, and C) standing clear of the lock entrance is not the appropriate response to the flashing amber light signal.