The only pure substance among the options is carbon dioxide (option D), as it has a consistent composition of carbon and oxygen. Soda, gasoline, and salt water are all mixtures and thus not pure substances. A pure substance is defined as having a uniform and definite composition without variations.
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Among the options given, carbon dioxide is the only pure substance. Soda, gasoline, and salt water are all mixtures that can vary in composition. Pure substances have a consistent composition and properties. ;