"Bye and thanks!"
Translation:Salut et merci !
I'm confused by that, too. I know that in Canadian French bonjour can be used for both. Probably salut is the same way. But in France, they do not use bonjour for goodbye, so they probably do not use salut for goodbye either. Because of this, I think this course is probably based on Canadian French. It's annoying, I know.
Adieu should honestly be accepted here.
Au revoir is used when you're planning to see the person again (re = again + voir = see).
Salut is an informal greeting used both as hi and bye.
And adieu is used when you don't necessarily plan to see them again, so it could be used in this context.