The correct answer is A) Number, position and angle of lashings. The number, position, and angle of lashings used to secure cargo on Ro-Ro (roll-on/roll-off) vessels are the primary variable factors that determine the initial lashing requirements. These factors influence the overall strength and effectiveness of the cargo securement system, which is crucial for safe transport. The other options, such as ABS requirements, port of origin, and vessel's draft, are not the primary variables that affect the initial lashing requirements on Ro-Ro vessels.