The correct answer is B) Ship's characteristics and motion in a seaway. The ship's characteristics, such as size, weight, and design, as well as its motion in a seaway, are the primary factors that affect lashing requirements aboard Ro-Ro (roll-on/roll-off) vessels. The lashing systems must be designed to secure vehicles and cargo effectively, taking into account the vessel's stability, acceleration forces, and potential movements in rough seas. Factors like wide variations in air temperature (A) or the vessel's trim (C) have a lesser impact on lashing requirements compared to the overall ship characteristics and motion.