Final answer:
Elizabeth laughs off the idea of Mr. Collins proposing.
Step-by-step explanation:
When Elizabeth's mother hints that Mr. Collins may be planning to propose to her, Elizabeth laughs it off and dismisses the idea. According to the provided discussions and conversations, Elizabeth reacts with amusement and does not express any interest in marrying Mr. Collins. Therefore, the correct answer is B) She laughs it off and dismisses the idea.