The last option seems to be the correct answer: Mrs. Linde resolves the conflict by committing to a new life with Krogstad, which develops the theme that new beginnings are always possible.
Krogstad is blackmailing Mrs. Linde's friend, Nora Helmer. He works at the same bank as Nora's husband and agreed to lend Nora some money. She needed the money to pay for a trip to Italy, where her husband recovered from a disease. Nora, however, forged her own father's signature to be able to borrow the money. Nora's husband means to fire Krogstad, so he is blackmailing her into convincing her husband to not do so.
In Act 3 of the play, it is revealed that Mrs. Linde and Krogstad have had a romantic relationship before. They agree to start a new life together. Mrs. Linde asks Krogstad to stop blackmailing Nora, but she also tells Nora to tell her husband the truth. Krogstad agrees to that, which mean Mrs. Linde resolves the conflict developed in the play and, by starting a new life with Krogstad, the theme that new beginnings are possible is developed.