In volume III, how does Lady Catherine attack Elizabeth for interfering with her plans with Darcy and how does Elizabeth respond to this?
Lady Catherine actually arrives in her coach to dictate to Elizabeth what she should do. She has heard a rumor that Elizabeth and Darcy are engaged and takes it upon herself to forbid the marriage, saying Elizabeth must back out because she is not a suitable bride for Darcy. Further, Lady Catherine says, it has always been intended that Darcy marry her own daughter.
As we know by now, Elizabeth has a temper and is not one to take insults lying down. She informs Lady de Bourgh in no uncertain terms that she is a gentleman's daughter and thus as suitable as Lady Catherine's own daughter to marry Mr. Darcy. Further, she tells Lady Catherine that she has no right to interfere in her marriage and that both she and Darcy can marry as they choose.
This is quite the affront to Lady Catherine, who feels she has the authority to tell everyone what to do all the time and be obeyed. Nevertheless, we learn that after Elizabeth and Darcy wed, Lady Catherine reconciles herself to Elizabeth in order to be invited to Pemberley.
Lady Catherine specifically mentions the impropriety of Elizabeth's mother, the lack of family rank on that side, and the elopement of Lydia as reasons that forbid the connection between Elizabeth and Darcy. Elizabeth is sarcastic and elusive during this interview. She does not want to give Lady Catherine any information regarding her history with Darcy, and so sidesteps the questions. When directly asked if Darcy has proposed, Elizabeth shoots back, "Your ladyship has declared it to be impossible." Only when asked if they are engaged does Elizabeth relent and confirm that they are not, but she will not make any promises to Lady Catherine, and actually ends the interview herself.
Lady Catherine de Bourgh comes to Longbourn to try to break up what she suspects is a relationship between Elizabeth and Darcy. She is extremely rude, and insists that Miss de Bourgh has been promised to Darcy since infancy. She belittles the Bennets’ worth, and chastises Elizabeth for her sharp tongue. Elizabeth tells her to mind her own business. She, as has been shown throughout the novel, is not afraid of Lady Catherine.