Answer:
A. Helmer knows that there is a piece of mail from Krogstad in the box.
C. Helmer thinks that Nora is nervous and dismisses her pleas about the mail.
D. Mrs. Linde fails to find Krogstad so he can take back the piece of mail.
Krogstad is blackmailing Nora. Krogstad works for Helmer, Nora's husband, but Helmer has decided to fire him. Since Krogstad knows about an illegal loan Nora has taken from the bank where they work, he is threatening to reveal that in case Nora does not convince her husband to give him his job back.
In Act 2, Nora is desperate because she knows there is a letter from Krogstad to her husband in the letter box, and so does Helmer. She pleads with him to not read any mail until after a party the next day, but Helmer thinks she is doing this out of nervousness and dismisses her pleas. To make matters worse, Nora's friend, Mrs. Linde, fails to contact Krogstad and ask him to retrieve the letter. All of that is evidence that support the prediction that Helmer will learn the contents of the letter.
