The correct answer is A) pressure-vacuum valve. Cargo tanks must be fitted with pressure-vacuum valves to prevent over pressurization when loading liquid petroleum products. These valves allow the tank to vent excess pressure and admit air to prevent the tank from collapsing during unloading. This is a regulatory requirement under the Code of Federal Regulations (CFR) for tank vessels carrying these types of cargoes. The other options are incorrect. Ullage openings are used to measure the liquid level in the tank, equalizing lines are used to balance pressure between tanks, and overpressurization valves are a less common alternative to pressure-vacuum valves for this application.