Answer:
sarcastic
Step-by-step explanation:
In this part of the book "Romeo and Juliet" the Nurse is telling Juliet that Romeo was banned from the city and for that reason, it would be better for Juliet to marry again because even if the second marriage is bad it will be better than marriage with Romeo, since he can not go back to town. At that moment Juliet is horrified at the words of the Nurse, who knows that Juliet loves Romeo, and asks if the nurse is speaking with her heart (speaking sincerely). The nurse says she wants her heart and soul to be cursed if she does not speak honestly and for this reason Juliet sarcastically replies "Amen."
Juliet replies with sarcasm because she wants to show that she does not believe that at Nurse's suggestion of marrying a man she does not love and insinuating that this man will be better than romeu. Juliet does not want to accept this "sincerity."