When one uses तू - the sentence would be framed this way " अब तू आराम कर " This is colloquial. Like in French one gives permission to "tutoyer". तू is even shorter than तुम and both are correct. But the sentence modifies accordingly. Hope this helps.
It seems ambiguous that the sentence is looking for the imperative here.
अब तुम आराम करे हो would seem to be a perfectly valid translation, if the intended meaning is "Now, you (do) rest" as opposed to "Now, you (must) rest"