<ol>
<li>The reason first novels are so often their writers' best work (is that it draws upon) all the experiences of childhood.
Answer is B. (is that these first efforts draw upon). Why?!?!?</li>
</ol>
<p>It’s an agreement issue. “Novels” is plural, and hence the word “it” does not agree.</p>
<p>You might think “it draws” could be replaced with “they draw” but then we have an ambiguity (between who or what is drawing on the experiences).</p>