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

The longest time interval of the ones that you've provides as possible answers would be an eon. 

So an eon is the correct answer.

This is because, when we're talking about time, a century denotes a time span of 100 years. A period usually doesn't denote a specific time interval but can be variable. However, from a time unit perspective, a period is under an era and an eon. It usually lasts around tens to one hundred million years. An era, on the other hand, lasts around seveal hundred million years. And an eon, finally, lasts around half a billion years or more.