Final answer:
The word 'malevolent' in the given sentence is an unobservable label because it describes a quality that is not directly measurable or visible.
Step-by-step explanation:
In the sentence 'Millicent is malevolent,' the word malevolent is an unobservable label. An observable behavior refers to an action that can be seen or measured, while an unobservable label is a descriptor that indicates a quality or characteristic that cannot be measured or directly observed just by looking. In this context, malevolence refers to having or showing a wish to do evil to others, which is a personality trait that one cannot measure or see but can deduce from behavior over time.