It is a little bit different in Italian than English. In Italian, most nouns have some variation of "the" in front of them; obviously, it is not that way in English. Therefore, when you translate the Italian sentences into English, you do not have to write "the" in front of them unless it is necessary for the sentence.
If you're taking dictation, then you have to type exactly what the recording says, not even a slight variation with the same meaning. If you're doing multiple choice, then you have to mark both of them. If you're translating from English, then "Il leone beve acqua." should be accepted; report it if it's not.
Well the article would be necessary in a sentence like "The lion drinks THE water." But in the Italian they use articles a lot more often than we do in English. Therefore, I think if there's a "the" in the English, it should be in the Italian, but if there's not a "the" in the English, then you can choose whether to put it in the Italian or not.