The question seems to be ''What does the author explicitly say can be found in the voice of a character?''
Well, firstly, what does explicitly means? Explicitly means in a clear and transparent mannerism. Thus this question is asking us, what does the author clearly mean and state about what could be found in a voice of a character?
Let's look at answer B, 'place'. Well, it says in the beginning of the excerpt that, "Yet deeper than character, or even place, is another concept: voice.''
The question asks what the author defines that can be found in a voice of a character. So, why would it be place if the author explicitly defines that the voice is deeper than any given place? Exactly.
Following that sentence, it states that it's simply more than any doorway... So that throws doorway out the same way 'place' was. The author says it's MORE to it. It isn't merely a doorway or a place.
And lastly, we have ancients. Actually, ancients isn't the answer to this problem. Ancients actually GIVES us an answer. Ancients believed in what? Ancients ideologies of a voice within a character was a SOUL. Correct? What is a synonym of 'soul'? Essence, spirit, spiritual, ect.
Therefore our answer would inevitably be Spirit. This sentence also backs up our conclusion:
''That is wholly appropriate, for in the breath—the voice—of a character lies its essential spirit.''
You were correct. The answer is in fact, 'D'.