Answer:
B. Is simply an extension of the title.
Explanation:
While a subtitle can inform the reader of the topic of the text or give more information about the title, its main purpose is to be an extension of the title.
Talk about how many have come through the years.
For example: Many immigrants have entered (whatever country, I'd assume America but you can fill in the blamk) but by far the most that have entered would be (number here) and (this percent) have stayed and.....
And if not that, maybe talk about how their cultures have affected the world or another country because of immigration. I hope that helps!