Keeping in mind that dramatic irony is a situation that the reader or audience is aware of but the characters are not, these lines are ironic because Juliet is unaware that as she speaks these lines, Romeo is standing in the shadows of her orchard garden listening to every word she says.
Dramatic irony is when the reader knows something critical that the characters do not. More specifically, in Romeo and Juliet, the reader already knows that Romeo has jumped over the wall of the Capulet orchard. At the beginning of Act II, scene ii, the reader first hears Romeo’s monologue disclosing his inner most feelings as he watches Juliet at her window (lines 1-25). Because of this monologue, we are made aware that Romeo is already watching Juliet when she says, “O Romeo, Romeo! Wherefore art thou Romeo?” in line 35. This is where the dramatic irony comes. We, as the readers, know something that Juliet does not.
Dramatic Irony is a technique used by playwrights wherein the audience knows a critical piece of information before the characters on the stage are aware of it. In the scene you describe, Juliet is calling for Romeo, but not expecting an answer. She calls his name out loud, though she's really only talking to herself, and she's wondering where he is at that moment. The dramatic irony is that he is hiding in the bushes listening to her speech. The audience knows he is there because they can see him, but Juliet is unaware of his presence.
I agree that dramatic irony occurs because Juliet does not know that Romeo is under her balcony, but what some answers to the question fail to realize, is that when Juliet says
"O Romeo, Romeo! Wherefore art thou Romeo?"
Juliet does not mean where are you Romeo, but instead is saying why is your name Romeo, because Romeo is a Montague and she wishes him to not be her family's enemy. She continues by saying
"Deny thy father and refuse thy name;"
It is an example of dramatic irony for one reason. Juliet is up on her balcony crying Romeo where are you? We the audience however know that he is underneath the balcony listening to her. This is dramatic irony
It is an example of dramatic irony because Romeo is there at her balcony and she has no idea.
You first need to understand that that wherefore in Elizabethan English means why. In this context,it is ironic because Juliet doesn't understand that she could fall in love with her enemy. If you read further she explains this.
It is also considered ironic because Juliet does not know Romeo is hiding beneath her balconey. Many teachers will consider this the correct answer if they have little knowledge of the language of the time.