I had a look in the lesson and in the quiz. I can't see any mistakes anywhere. If it does happen again, please report the issue via 'report it' button in your correction board.
'jus' is masculine and therefore any article in front would be in the masculine form:
un jus de raisin = a grape fruit juice
le jus de la viande = the juice of the meat
du jus de pomme = some apple juice
See other examples here: jus
I hope this is helpful.
Bonne journée !
Because it is in the masculine form
de la jus is correct
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