Answer: a
Eon, Era, Period
Explanation:
There may be 2 or more Eras in an Eon, and at least two Periods in an Era.
For example, The Phanerozoic Eon consists of the Cenozoic, Mesozoic, and Paleozoic Eras.
The Mesozoic Era consists of the Cretaceous, Jurassic, and Triassic Periods.