The correct answer is D. Eons.
Explanation
Geochronological units are divisions of time used in historical geology to form the geological time scale. These units have as basic unit age and are grouped, in increasing order of duration, in times, periods, ages and eons.
Despite the proposal made in 1957 to define an eon as a unit of time equal to one billion years, the idea was not accepted as a unit of measure itself and is rarely used to specify an exact period of time, but rather It is used as a large but arbitrary amount of time. An example of eon is the Phanerozoic eon that is divided into three ages: the Paleozoic, Mesozoic and Cenozoic; likewise, the Cenozoic era divided into three periods that are Paleogene, Neogene and Quaternary. So, the correct answer is D. Eons.