Final answer:
Juliet responds with surprise and defiance to Lady Capulet's announcement of her planned marriage to Paris, leading to a conflict between her desires and her family's expectations.
Step-by-step explanation:
When Lady Capulet informs Juliet of the planned marriage to Paris in Act 3 of Shakespeare's play, Juliet's response is one of surprise and defiance. She is not agreeable to the marriage, and her refusal to comply with her parents' wishes signals the beginning of a significant conflict between her individual desires and her family's expectations. Juliet's reaction is pivotal to the development of the plot, as it furthers the tension and ultimately leads to the tragic events that follow.