D. Susan is what is known as an "addlehead" It's a fairly common collocation to say that someone or is something "is what is known as" some unusual or obscure word.

Jonathan gave a good answer. D. Susan is WHAT is known as an "addlehead."

I would say the answer is "A", based on matching the grammar like this: Who is known? Susan is known. But the question is not clearly defined by the paper, from what I see here. And I can't read the Chinese, I'm only talking about the English part.