A pure substance is one which contains molecules of just a single type. If a substance contains molecules of 2 or more kinds, then it is a mixture. Among the given options, only carbon dioxide is a pure substance. Carbon dioxide consists of only one type of molecule: CO2, formed by the combination of carbon and oxygen atoms in a 1:2 ratio. The other options consist of more than one types of molecule. Gasoline, for example, is a mixture consisting of isooctane, butane, MTBE, and others. Soda and salt water are also mixtures.
Elements are the purest form of substances, since they contain atoms of only 1 type (excluding the isotopes). Some examples of elements include iron, aluminum, etc. Carbon dioxide, water, etc. are compounds and are made up of two or more elements, combined in a fixed ratio.
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