I wonder why "Which is the professional" was marked as incorrect. I could imagine saying it that way in English.. Doesn't ποιος also mean "which"? Or because professional implies a human being, we are to use who instead?
Ποιος/ποια/ποιο are used for both 'who' and 'which one'. Τι is used to literally ask 'What is this?' Saying "Τι είναι ο επαγγελματίας;" means 'What does ο επαγγελματίας mean?'
I wonder about this situation: you're shown a picture of someone next to simple, poorly constructed pottery dish and another next to a complex, well-made dish. In English you might be asked "which is the professional". In Greek would you use "ποιος είναι ο επαγγελματίας" in this situation or would you ask it differently?