The correct answer is D) Size and weight of vehicle/cargo unit. The size and weight of the vehicle or cargo unit is a key variable factor that affects the initial lashing requirements aboard Ro-Ro (Roll-on/Roll-off) vessels. Larger and heavier vehicles/cargo require more robust lashing arrangements to secure them in place during the voyage and prevent shifting or movement. This is directly specified in the regulations governing Ro-Ro vessel cargo securing, as the lashing requirements are based on the size, weight, and other physical characteristics of the cargo units. The other options are incorrect because they do not directly impact the initial lashing requirements in the same way. The shipper's reputation, tire pressure, and age of the vehicle/cargo are not the primary determinants of the lashing needed, whereas the actual size and weight are the critical factors.