The answer is shown above
Your thesis is the main point of a composition
I'm not 100% sure on this one, so I'm just gonna go through the list!
"Juliet's demand that Romeo constantly send her messages is so unrealistic that the audience forsees she will be disappointed." I don't see how that builds tension, so this one can be eliminated.
"Romeo and Juliet's love for each other is so deep and pure that any audience would know problems will arise." I don't see how this builds tension either, only if the reader is pessimistic. I think this one can be eliminated too.
"The Nurse's sudden entrance, communicating a sharp warning, alerts the characters and audience to danger." I feel that this one very clearly signals tension, so this is an option.
"Romeo and Juliet's disagreement about whether the bird is a nightingale or a lark is symbolic of the unknown threat that approaches." I can see how this one can build tension as it is a bit of an argument/disagreement, but I don't think it creates more tension than the Nurse, so I'd go with your third option, "The Nurse's sudden entrance, communicating a sharp warning, alerts the characters and the audience to danger."!
Hope I helped!
(tl;dr: the answer should be the third one aka the one that mentions the nurse) :D
