"The girl eats fruit."
Translation:La ragazza mangia la frutta.
The Zingarelli dictionary gives several uses of the definite article including the English typical use of one definite thing but also it indica e determina una specie, una categoria, un tipo (to indicate and determine a species, a category, a type). So we would say the president of italy but not the avarice or the fruit. Italian can use the article for all of them. That said, as the lessons progress Duo gets less picky about this.
I think that 'della frutta' has the sense, 'of the fruit' meaning, 'some portion of the fruit that exists in the world.' It's used in English (to eat 'of the fruit') but it's more archaic/poetic. For example, you'll find in English Bibles, Jesus saying, 'I will not drink of the fruit of the vine' meaning 'I will not drink any wine of all wine produced from grapes.' But since we don't speak/think that way anymore in English, it can be hard for English-speakers to 'feel' it when another language uses 'of the' form meaning something like 'any' or 'some of' the fruit (available in the world).
Sometimes in English we can use another verb to help us understand the sense of 'of the + noun' in other languages. For example, we say, 'I eat (food)', but we 'partake of (food).' Someone could say to you, 'Will you partake of the shrimp?' It sounds a bit old-fashioned or jocular, but it would be understood to mean, 'Will you eat any of the shrimp?' Or suppose the shrimp was bad and made people sick? You could say, 'She partook of the shrimp' meaning, 'she ate SOME OF THE shrimp.' So there's still some phrases in English that have their roots in the more archaic forms of 'of the' instead of just 'the,' as you find 'della' in Italian.
'The girl eats of the fruit' means, 'the girl eats fruits of some sort.'
'The girl eats fruit' implies that 'fruit' is a food group the girl partakes of.
'The girl eats THE fruit' means, 'some specific fruit that the speaker and listener both have in mind, both can identify as the particular fruit the girl eats.
'Lei' is simply, 'she.' They are testing for you to know the words for 'the girl' not the pronoun that replaces 'the girl.' We would only use 'she' in a conversation when 'she' has been identified already. So if you are in a context, then you can replace 'the girl' with 'she.' But there's no context here.
If you just say - out of the blue - 'she eats fruit', your listener will say, 'Who? Who eats fruit?'
But if you say, 'That's my sister; she eats fruit,' then we know that 'she' refers to 'my sister.'
Or 'The girl is here. She eats fruit.' Then we know that 'she' refers to 'the girl.'
We use 'she' to REPLACE any female person, but we can only sensibly do that in a context when the person has already been identified.