Final answer:
Sylvie and Monique would say 'Au revoir! A tout à l'heure!' to say goodbye after lunch and when they will see each other again the next day in class.
Step-by-step explanation:
Sylvie and Monique would use the expression Au revoir! A tout à l'heure! to say goodbye to each other after lunch and when they will see each other again the next day in class. This expression translates to 'Goodbye! See you soon!'. It is a common way to say goodbye in French when you expect to see the person again soon.
Learn more about French greetings