Answer:
It should be
C. What's the point of this piece of writing?
Explanation:
Because you are asking for the main idea, lets look at the answer choices:
A) Who said this quotation?
No - because that is one part of the writing, unless you are writing about a single person and the quote is said by he or her. But that doesn't matter, because you don't always need to know this piece of information.
B) Where can I read more on this subject?
No - because that is about another topic, not the single piece of writing.
<u><em>C) What’s the point of this piece of writing?</em></u>
<em>Yes - because you are talking about the writing and that's the only that makes sense. Plus this will work for almost every kind of writing, narrative, fiction, informative, etc.</em>
D) How long is this story or essay?
No - because it has nothing to do with the information that the piece of writing is providing.