Final answer:
The best evidence to classify an experiment as a physical or chemical change is option (a) 'The substance changed color and produced a gas,' since it indicates that a new substance has been formed, characteristic of a chemical change.
Step-by-step explanation:
The student is seeking to distinguish between a physical and a chemical change. An option that indicates a chemical change might include a change in color, the production of gas, or a change in temperature because these suggest a change at the molecular level, where new substances are created. Among the given options, (a) 'The substance changed color and produced a gas' offers the best evidence for a chemical change. This is because color change and gas production often indicate that a new substance has been formed through a chemical reaction, as opposed to just a change in physical state.
To elaborate, physical changes are generally reversible and do not result in the production of new substances; they include changes in state, shape, and temperature that do not involve altering the molecular composition of the substance. On the other hand, chemical changes result in one substance becoming another and are often accompanied by energy changes, color changes, production of gas, or formation of a precipitate, indicating that a chemical reaction has occurred.