The correct answer is "To develop a contrast between the two sons of Madame Lebrun".
Mrs Potellier and Madame Ratignolle are very opposite characters in terms of their attributes even though they're friends. Each of them represent distinct female roles. Pontellier isn't very close or warm to her children and makes them do chores quite often. Ratignolle, on the other hand, displays very strong maternal instincts and is very kind to them, that's why they're so joyous as soon as she appears.
Hope this helps!