In what form must natural gas be in to be classified as liquified natural gas (LNG)?

Natural gas is classified as liquefied natural gas (LNG) when it is in a liquid state. This process involves cooling the gas to very low temperatures, typically around -260°F (-162°C), which transforms it from a gaseous form into a liquid. This liquefaction process significantly reduces the volume of natural gas, making it easier to transport and store.

When natural gas is in its gas state, it is not classified as LNG because LNG specifically refers to the liquid form. The solid state is relevant in other contexts but does not pertain to natural gas or its liquefied version. Similarly, vapor state indicates that the substance is still in a gaseous form and has not undergone the necessary cooling to become liquid. Therefore, the classification of LNG is strictly tied to its status as a liquid, which is why the answer is correct when identifying that natural gas must be in a liquid state to be classified as LNG.