The correct answer is C. For the first time in decades, Baby Boomers have been outnumbered by another group--the Millennials.
A hook is the sentence that metaphorically 'hooks' you - that is the most engaging sentence in the introductory paragraph that makes you interested in what is going to happen next. In this paragraph, option C is the hook because we are about to find out how the Millennials managed to replace Baby Boomers.