Final answer:
A gas generated when two liquids are mixed together is an observation of a chemical change, as it represents the formation of new substances with different properties than the original reactants.
Step-by-step explanation:
Among the given options, the observation that indicates a chemical change has occurred is C: A gas is generated when two liquids are mixed together.
This is an example of a chemical reaction wherein the original substances interact to form new products with different chemical properties, often with a release of gas, energy, or a color change. Examples of such changes include the burning of methane to produce water and carbon dioxide, or the reaction between vinegar and baking soda to release carbon dioxide gas.