Translation:Thank you, sir.
I can see why duolingo staff wouldn't have included it in the acceptable answers as it's kind of regional and not exactly the same as the dutch, but could one interpret this (not directly translate) correctly as "thank you kindly, Sir?" I instinctively want to find something to express the intensifier for wel rather than just saying thank you.
Dank u wel simply means thank you. Adding more to it than that means you are over translating the wel part.
Yet "Dank u, meneer" (without the wel) exists too. I don't think it's necessarily overdoing to it to translate "Dank u, meneer" (= "Merci bien, monsieur") as "Thank you very much, sir".