"Are they at work?"
Translation:Er de på arbejde?
While this is true, they suggest that it should be "Er de på job?" which translates to "Are they at their workplace" which in short may be shortened to "Are they at work?". This being said, I see no reason why you shouldn't be able to say "Er de på arbejdet?" however I'm Swedish so danish grammar is still a little difficult to understand
If what zeyneparman says is correct, then "Er de på arbejdet?" would be asking if they are at work in the sense that they are at a particular location, rather than in the sense that they are engaged in an activity. Both would be correct translations of the English sentence, as fas as I can see.