Substances are matter that has uniform and constant composition: same number and kind of atoms bonded in the same way.The substances are compounds or elements.From the options given, only silver is a substance, because it is an element, and all silver materials have only silver atoms. Air is a mixture of different gases,which may varies in composition; gasoline is a mixture of liquids which may change in composition; steel is an alloy (a mixture of compounds and elements) which varies in composition. So, the answer is Silver.