"I read a book during the meal."
Translation:Leggo un libro durante il pasto.
It is helpful to keep in mind that we have not learned past tense yet, so all of these phrases should be read as present tense.
Because mentre is a conjunktion and durante is a preposition. And conjunktion - as far as I know - combine two sentences. So (and that's more like a guess) I would say, if you want to use mentre here, the sentence would have to be: "Leggo un libro mentre mangio il pasto" - "I read a book while I eat". I'm not native English but I just learnt, that it's the same in english. During is a prep and while is a conj.
I am doing a placement test as an "advanced learner," and incorrectly assumed this was testing past tense knowledge, and answered as such. Perhaps this should be indicated somehow when sentences are being used for placement tests.