"Is this their juice?"
Translation:זה המיץ שלהן?
If we were simply asking "Is the juice theirs", we would say המיץ שלהן? or האם המיץ שלהן?; however since we are asking "Is this their juice", we say זה המיץ שלהן? or האם זה המיץ שלהן. Knowing how to correctly use זה in such contexts can be a little tricky. It depends on what you want to emphasise. For instance, if we wanted to ask "Is this juice theirs?" we would say המיץ הזה שלהן?, giving the juice a definite article.
Because it's a different structure of sentence, in Hebrew as well as in English.
Is this juice theirs? = המיץ הזה שלהם?
In most contexts the exercise's sentence and this one can be interchanged, but they have slightly different emphasis and there are some contexts in which only one of them will work.