The correct answer is D) transport vehicles. Ro-Ro (Roll-On/Roll-Off) vessels are designed specifically to transport wheeled cargo, such as automobiles, trucks, and other vehicles. The primary objective of a Ro-Ro vessel is to efficiently load and unload vehicles by allowing them to be driven on and off the ship, rather than using cranes or other cargo handling equipment. The other answer choices are incorrect because they do not accurately describe the primary purpose of a Ro-Ro vessel. Ro-Ro vessels are not primarily used for transferring containers without chassis (A), loading the vessel with palletized cargo (B), or being loaded full and down to her marks (C), which are more characteristic of other types of cargo ships.