The correct answer is B) Chain lever. Chain levers are the tensioning device commonly used to secure heavy vehicles aboard Ro-Ro (roll-on/roll-off) vessels. They allow the operator to apply high tension to the chain securing the vehicle, ensuring it remains firmly in place during transit. This is in accordance with regulations and best practices for the safe loading and transport of vehicles on Ro-Ro ships. The other options, such as buckle tensioners, ratchet tensioners, or adjust-a-matic tensioners, are not typically used for this specific application on Ro-Ro vessels, where the high tension and security requirements call for the use of a chain lever.