Final answer:
You should say 'Au revoir' to bid your mother goodbye as she leaves for her trip to China.
Step-by-step explanation:
As your mother is leaving for a two-month trip in China, you would say 'Au revoir' to bid her goodbye. This is a common French phrase used to say 'goodbye' when someone is leaving. It is polite and appropriate in this context.
Learn more about Saying goodbye in French