Which best uses the best interogative pronoun to complete this sentence

An interrogative word or question word is a function word used to ask a question, such as what, when, where, who, which, whom, whose, why, and how.  They are sometimes called wh-words, because in English most of them start with wh-.

A pronoun has been theorized to be a word that substitutes for a noun or noun phrase. It is a particular case of a pro-form.

Since the sentence ins't provided here, a correct answer cannot be given.

The hook is
B. For the first time in decades, Baby Boomers have been outnumbered by another group--the Millennials
This is a hook because when you see the 'first time in decades' it really catches your interest.
Hope this helps!