The correct answer is B) Turnbuckle. Turnbuckles are the proper tensioning device used with chain to secure heavy vehicles aboard Ro-Ro (roll-on/roll-off) vessels. Turnbuckles allow the tension in the chain securing the vehicle to be easily adjusted, which is crucial when dealing with the variable forces exerted on cargo during a maritime voyage. The other options, while valid tensioning devices, are not the industry standard for securing vehicles on Ro-Ro ships.