"What is the advantage" should be accepted. "Which is the advantage?" Is not correct.
'What is the advantage' is shown as an alternative. I don't like "which" but there may be a minimal use so we left it as a n alternative.
Ok, there are two issues here. One is that the hints are like dictionary definitions. Not all definitions are suitable in all cases. So, when choosing a word from the hints it's best to choose the first one...that's how Duo is set up.
However, we do have "profit" used in a sentence...and it's our fault that the sentence was: "Which is the profit?" and should also have been "What is the profit?" We have corrected that and apologize for the slip-up.
Frankly, I don't like the use of "profit" although technically it's correct it sounds unnatural.
So, to make things easier why don't you just chose the first hint which is most often the correct one and always more authentic.