The correct answer is B) Buckle or ratchet tensioner. Ro-Ro (roll-on/roll-off) vessels are designed to transport wheeled cargo, such as light vehicles. To secure these vehicles during transit, buckle or ratchet tensioners are commonly used. These devices allow the cargo to be tightly fastened to the ship's deck, preventing any movement or shifting during the voyage. Chain, manila rope, and wire are not the primary methods used for securing light vehicles on Ro-Ro vessels, as they may not provide the necessary level of stability and tension required for this type of cargo.