Final answer:
The longest chronologic time unit represented on the geologic time scale is the eon.
Step-by-step explanation:
The longest chronologic time unit represented on the geologic time scale is the eon. Geologic time is divided into four eons: the Hadean, Archean, Proterozoic, and Phanerozoic. The Phanerozoic eon is the most recent eon and is further divided into eras, periods, epochs, and ages.