The correct answer is A) Turn on ventilation for cargo holds. After lowering the ramp on a Ro-Ro (roll-on/roll-off) vessel, the first priority is to ensure proper ventilation of the cargo holds. This is a crucial safety measure to prevent the buildup of dangerous fumes or lack of oxygen, which could pose a serious risk to personnel during cargo operations. The ventilation system must be activated before any cargo is loaded or unloaded to create a safe working environment. The other options are incorrect because they do not address the primary safety concern of proper ventilation. Calling out the deck department (B) or turning on a cargo light (C) are not directly relevant to the immediate steps required after lowering the ramp. None of the above (D) is also incorrect, as turning on the ventilation is the appropriate action.