The correct answer is D) heavy. The main disadvantage of using chain lashing on heavy vehicles aboard Ro-Ro (roll-on/roll-off) vessels is that chain lashing is a heavy method of securing cargo. Compared to other lashing methods like web straps or synthetic ropes, chain lashing adds significant weight to the overall cargo, which can impact the vessel's stability and loading capacity. This makes chain lashing a less desirable option for securing heavy vehicles on Ro-Ro ships, where weight considerations are critical. The other answer choices are incorrect because: A) temperature has minimal impact on the effectiveness of chain lashing, B) chain lashing is generally quite durable and resistant to damage, and C) the strength-to-cost ratio of chain lashing is often favorable, though the overall weight may be a greater concern.