The right answer is the second one: Serious and authoritative. Hermia herself addresses him as "your grace" and she contrast her "modesty" with his "presence." Furthermore, Hermia's father, Egeus, requests him to let him exercise the right to force her to marry the man he wants or to have her killed. Moreover, Theseus courted Hippolyta violently, forcibly, as he himself recognizes, and he also has the authority to somehow force the young people of Athens to celebrate their union with them, and to do it "with extravagant festivals and public festivities." These words and actions indicate that Theseus is an authoritative and powerful man. In addition, when he argues with Hermia about considering marrying Demetrius rather than Lysander, since that is her father's wish, he sounds serious and categorical: she will die or she will never see a man again if she disobeys him, since that's the law.