The correct answer is D) The cargo tanks are used for one type of cargo only. The reason this is the correct answer is that LPG (liquefied petroleum gas) cargo tanks are typically dedicated to a single type of cargo, such as propane or butane. Since the tanks are not used to carry multiple, potentially incompatible cargoes, there is no need to gas-free the tanks between voyages. Gas-freeing is primarily required when cargo tanks need to be cleaned or inspected, or when the tanks were previously used for a different type of cargo that needs to be removed before loading a new cargo. The other answer choices are incorrect because: A) LPG is a flammable gas, not high in oxygen content; B) LPG may not be compatible with all other cargoes; and C) the inspection frequency for LPG cargo tanks is similar to that of oil tankers.