The correct answer is D) A substance that at normal temperature and pressure would be a gas. A liquefied gas is a gas that has been compressed and cooled to the point where it becomes a liquid. At normal temperature and pressure, these substances would exist in a gaseous state, but they are stored and transported as liquids to make them more compact and easier to handle. Examples of liquefied gases include propane, butane, and liquefied natural gas (LNG). The other options are incorrect because they do not accurately describe the defining characteristics of a liquefied gas. Option A refers to a gas mixed with another substance, option B describes a liquid that must be heated to become a gas, and option C describes a liquid that must be cooled to absolute zero, which is not the case for typical liquefied gases.