The correct answer is B) flammability. Liquefied petroleum gas (LPG) and liquefied natural gas (LNG) are highly flammable substances, and this is considered their primary hazard. When these gases are released, they can form explosive mixtures with air, posing a significant fire and explosion risk. The high flammability of LPG and LNG is a key consideration in their safe handling, storage, and transportation, as outlined in the regulations governing the use of these fuels. The other options, such as toxicity (A), pressure (C), and temperature (D), are also important safety considerations, but they are not the primary hazard associated with LPG and LNG. Flammability is the most critical characteristic that must be carefully managed when working with these liquefied gases.