La fille mange de la soupe
Every exercise has the same construction: manger le pain, manger la soupe. So if I need to translate: the girl eats soup, it shout be La fille mange la soupe. The only accepted answer is: la fille mange de la soupe. I think this translates into: the girl eats from the soup.
It's tricky: for an undefined quantity of something, in English you can say either "The girl eats soup" or "The girl eats SOME soup" without any major difference in meaning. However, in French, you MUST use the construction "du/de la/des" (depending on gender and number) to say "some" of something. Try saying the English sentence with "some"; if it works, you need to use "du/de la/des". ex: Je mange du pain = I eat (some) bread; Je vois des gens = I see (some) people.
manger la soupe = to eat the soupe
la fille mange de la soupe = the girl eats some soup
la fille mange dans la soupe = the girl eats from the soup
In English we drop the use of some. We can say "the girl eats soup" as an undefined quality of soup, but in French you cannot drop all the same words we drop in English.
Remember that "le/la" means "the" and denotes a predefined item, where as "du/de la" is closer to saying "some" in English.
Mange de la soupe is: she eats soup. This soup is an unspecified soup. She eats any, or some soup. But if you want to know whether she eats one specific bowl of soup, you would use the definite article 'la'. I think the words 'definite' and 'indefinite' play a big part in this. That's how I remember this easily! :)
I have written an article on this subject which may interest you: http://spanishplus.tripod.com/french/Articles.htm#TopOfPage
"la fille mange dans la soupe" is very strange, literally "the girl eats IN the soup." I see her swimming in a large pool of soup, eating some bread.
If the girl eats all of the soup--there is a defined quantity and she is eating it all--you say "la fille mange la soupe". OR if you want to know if she EVER eats soup, you say "la fille mange la soupe?" also.