The correct answer is B) Center of gravity of vehicle or cargo unit. The initial lashing requirements aboard Ro-Ro vessels are primarily determined by the center of gravity of the vehicle or cargo unit being transported. The position of the center of gravity affects the stability and securing requirements, as it determines how the load will shift during the vessel's motion. Factors such as the vessel's draft, changes in humidity, or the age of the vehicle or cargo unit do not directly affect the initial lashing requirements.