The proper chemical name for natural gas is:

Natural gas is primarily composed of methane, which is a simple hydrocarbon with the chemical formula CH4. Methane is the predominant constituent of natural gas, typically making up about 70-90% of its composition. This makes it the correct chemical name for natural gas.

The other options listed represent different hydrocarbons that are found in smaller quantities in natural gas or are completely different substances. Propane, butane, and octane are also hydrocarbons, but they have different chemical structures and properties. Propane (C3H8) and butane (C4H10) can be derived from natural gas or crude oil but are often considered separate fuels with distinct applications. Octane (C8H18) is a component of gasoline and is not associated with natural gas. Therefore, methane is the definitive and proper chemical name for natural gas, indicating its predominant hydrocarbon content.