In my opinion, the answer is D, varying sentence lengths. Meaning that you should have simple, compound, and complex sentences, and vary them. Because you should always have specific examples, not general statements. Cliches should be avoided. And paragraph indentation isn't the crucial element in a paper.
I believe it is C), leaders. :)
The right answer is: "The nurse does not know that Nora will leave her children, but the audience does."
When Torvald humiliates Nora, he tries to apologize with her, but she has changed, she realized her husband is a very strange man for her and in her marriage was always a doll, so she decided the only way to recognize herself is leaving from the house.