"Which girls eat the chocolate?"
Translation:Quelles filles mangent le chocolat ?
I had the same question, and I had already asked it a few questions back. This question made me lose all my hearts, so I went to the discussions page of the "questions" topic. In there, there's a whole discussion on quel and lequel.
So, basically, what I got from the discussion page is that:
Quel - which; Lesquel - which one
Putting "Lesquelles filles" would be redundant since lesquelles is already an interrogative pronoun, and it would translate to "Which ones girls," which doesn't really make sense. Quelles, however, is an adjective in that it modifies the noun "filles," making the English translation "Which girls," making it have a lot more sense than "Which ones girls."
Err, of course, I could always be wrong, since I have no other experience in French other than lessons from Duolingo. I apologize if I made this more confusing for others!
"The chocolate" implies that there is some chocolate that you and the person you are communicating with are both already aware of... "a chocolate" means you are happy with any of the available chocolates. So it depends on context. Perhaps someone stole your chocolate, you would say "give my the chocolate" not "a chocolate"...