The longest interval of time is called
an eon
a period
an era
a century

The longest interval of time is called an eon.

An eon is said to be a unit of geological time equal to a billion years , or a very long period of time.

An example of an eon is the unit of measure used when talking about how long ago the solar system  created.

Another example is the amount of time since dinosaurs roamed the earth.

In comparison, an era is shorter than eon. For example the Paleozoic era was 542  million to 251 million years ago.

the answer is a. harriet tubman was a nurse on the battlefield.