B: What? Who is required to attend the meeting this afternoon?
Explanation:
The sentence in option B contains correct capitalization because <em>What? </em> is a separate and complete expression, asking for some confirmation. The next sentence begins with correct capitalization in <em>Who, </em>differently from sentence A. The sentence C is wrong because it doesn't have question mark, capitalization is used after a comma.
<em>"So the duke said these Arkansaw lunkheads couldn't come up to Shakespeare; what they wanted was low comedy—and maybe something ruther worse than low comedy, he reckoned."</em>