The correct answer is letter B) : Now that they are in the majority, there are many areas where Millennials may impact our society and priorities.
First of all, a hook in an introductory paragraph has the aim of capturing the reader's attention so that he or she wants to read what is coming next in the essay.
In this case,this sentence is exactly fulfilling that condition.The writer's objective is that the reader finds out what are the impacts on the society and the priorities that the Millennials may cause,which is the reason why the author is presumably writing this essay.
The rest of the sentences are in this introductory paragraph just to present the topic but they are not the focus of attention.