Which of the following describes a liquid mixture of complex hydrocarbon compounds used widely as a fuel source?

A liquid mixture of complex hydrocarbon compounds that is widely used as a fuel source is petroleum. Petroleum, also known as crude oil, is primarily composed of hydrocarbons that are extracted from beneath the Earth's surface. It can be refined into various products, such as gasoline, diesel, and other petrochemical products, making it a crucial energy resource globally.

Natural gas, while also a hydrocarbon fuel, is primarily in gaseous form and not a liquid mixture. Coal is a solid fossil fuel composed mainly of carbon and is not a mixture of hydrocarbons in liquid form. Biofuel is derived from biological materials and typically consists of organic compounds; it is not a complex hydrocarbon mixture like petroleum. Hence, petroleum stands out as the correct answer in this context.