b.That Romeo is listening to her
She is unaware that Romeo is down below her balcony listening to her to profess her love for him. While the audience can see Romeo, Juliet cannot. The dialogue from their conversation also confirms this. In their conversation (once he reveals himself), she tells him that she never meant for him to know she loved him. Well, not yet at least. She fears that by giving him this knowledge she made it too easy for him. When he asks for her to profess her love before he leaves. She tells him she already did before she knew he was listening.