You could say both but the meaning is slightly different -
When they arrive, you will do this , do that = Quand ils arriveront, tu feras ceci, cela...
as opposed to -
Once they have arrived, you will do this, do that = Quand ils seront arrivés, tu feras ceci, cela
But as the lesson and quiz referred to the futur simple after quand , arriveront was expected.
Hope this helps!
NOT A REAL REPLY.
I'm replying to this question myself to see if that will keep it in the system because the first time I submitted it the system deleted it.
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