Final answer:
The geologic time scale unit that covers the longest amount of time is the eon. Eons encompass eras, which are further divided into periods, epochs, and ages.
Step-by-step explanation:
The geologic time scale is a system used to describe the timing and relationships between events that have occurred throughout Earth's history. To answer the student's question, the geologic units that typically cover the longest amount of time are eons. Eons are divided into eras, followed by periods, epochs, and finally ages. Earth's history begins with the Hadean eon and the current eon is the Phanerozoic. The first three eons, Hadean, Archean, and Proterozoic, are sometimes collectively referred to as the Precambrian, representing a vast stretch of early Earth's history.