Which of these is the best definition of an era?

A period of time which was different, and that it made a mark throughout history and civilization. Like for example the Mesopotamian era. This specific era is has been mentioned that it is probably the beginning of human civilization.

there are a number of different ideologies that come into conflict with regard to immigrants and immigration to the us.

ask google there you might be able to find an answer for the question you're trying to ask