Well, if I met someone who I thought might be the author of something, I probably would not be on "tu" terms with them. I think "Sunteti autorul?" is more appropriate.
'Sunteți autorul' was the first thing I thought of when I saw this sentence with the same exact thoughts in mind. Unfortunately Duolingo didn't agree with me.. But I did report it.
The order of tu and ești should be interchangeable in the answer
"Tu ești autoarea" should be accepted as the female form.