Question

Which of the following is the shortest interval of geologic time?

an eon
a period
an era

Answer 1

The answer is Period. This is the shortest interval of geologic time.

Answer 2

An eon is a geologically around tens of millions of years. A period is a million year included in an eon which is also made up of several eras. Therefore my answer is a period due to it being a subset of an era which is a subset of an eon.