"Do you know what exactly this experiment is for?"
Translation:Tu sais à quoi sert exactement cette expérience ?
A word starting with a vowel.would never use ce. Cet and cette are possible.
Frankly, I don't trust your report anyway. I have never known Duolingo to make this type of error in two years. Even then, it would be implausible that thousands of people doing the exercise wouldn't notice it. I would need a link to believe your memory. Sorry, nothing personal. I just have to go with my experience.