Final answer:
Mr. Collins threatens to withdraw his offer of marriage to Elizabeth because he believes her family is not respectable enough.
Step-by-step explanation:
Mr. Collins threatens to withdraw his offer of marriage to Elizabeth after he talks to Mrs. Bennet because he believes Elizabeth's family is not respectable enough. In the novel, Mrs. Bennet represents the societal view that a woman's social advancement and financial security depend on marriage. By talking to Mrs. Bennet, Mr. Collins realizes that Elizabeth's family does not meet the social status he desires in a wife, causing him to reconsider his offer.