The correct answer is D) Norman Pins. Norman Pins are metal pins used to secure the towing connection between a tug and the vessel being towed. They help prevent the tug from tripping or capsizing by securing the towing connection and distributing the towing forces evenly. Norman Pins are a critical safety feature that prevent the tug from becoming unstable or overturned during the towing operation. The other options are incorrect because: A) Safety shackles are used to connect the towing line, but do not directly prevent tripping or capsizing. B) Under riders are not used to prevent tripping or capsizing, but rather to minimize the risk of the towed vessel going under the tug. C) Surge lines help control the motion of the towed vessel, but do not directly prevent the tug from tripping or capsizing.