Question

What substance is a mixture? Salt Gasoline Aluminum Carbon dioxide

Answer 1

Gasoline is made out of oils thus creating a mixture

Answer 2

Answer is: gasoline.

Gasoline is a mixture  of many different hydrocarbons: alkanes (paraffins), cycloalkanes  and alkenes (olefins).

Salt and carbon dioxide are chemical compounds, aluminium (Al) is an chemical element.

Pure substance is made of only one type of atom (element) or only one type of molecule, it has definite and constant composition with distinct chemical properties.

Pure substances can be separated chemically, not physically, that is difference between pure substances and mixtures.