Question

which is a pure substance? soda carbon dioxide gasoline salt water

Explanation:

A pure substance has a fixed composition and distinct properties. Soda is a mixture (contains water, flavoring, CO₂, etc.). Gasoline is a mixture of hydrocarbons. Salt water is a mixture of salt (solute) and water (solvent). Carbon dioxide ($\ce{CO_2}$) has a fixed chemical formula and is a compound (a type of pure substance), with consistent composition.

Answer:

carbon dioxide