The correct answer is B. Is longer than an era.
Step-by-step explanation:
The passage explains the different divisions geologists use to understand and explain the history of Earth. In this, it is mentioned the units used are "eons, eras, periods, epochs and ages", and these are organized from the largest unit to the smallest unit. This implies an eon is longer or comprises more time than an era, a period, etc. This idea is also explained in "while an eon is longer than an era, there is not set length" that shows eons are longer and this does not have a specific duration.