Healthy plants serve as the best empirical evidence for the cycling of carbon dioxide in a terrarium because they engage in photosynthesis, a process where they absorb carbon dioxide and release oxygen. If the plants are thriving, it indicates that they are effectively utilizing carbon dioxide, which suggests that the gas is cycling properly within the terrarium's ecosystem. In contrast, foggy air may indicate humidity but does not directly relate to carbon dioxide levels, and a lack of insects does not provide information about the carbon cycle.