How do scientist determine when an era begins and when it ends?

The way that scientist determine when an era begins and when it ends is : Mass extinction of an organism
For example, millions years ago, the world was under the era of dinosaurs, which were simply the most superior beings on earth during that time, after they became extinct, the earth enter the era of the mammals

This question is an opinionated one, and we don’t know anything about the module or anything, thus we can not answer it. but if you are having trouble on what it is asking basically it is just asking to (using your knowledge on the topic) say which topics you were most interested and which were the most challenging.hope this : p