how shall we guess that here it is used in Accusative and not Nominative case?
You can't possibly know that, in English those are the same. Doulingo should accept both Hungarian translations.
sure, if this is a part "Do you want a book or bread?" - it is easy to guess... but just "book or bread" it is tricky )) I reported this issue
Now it even accept one in nominative and one in accusative. Which it should, even though I can't think of a good English example. A bad example might be: "Do you want to read a book or is this bread getting mouldy? Book or bread?"
A few months ago it actually was "könyvet vagy kenyéret" - they seem to have changed it for whatever reason
I don't know about it but "kenyéret" makes no sense and it should be "kenyeret".
I wrote: Egy könyv vagy kenyér?
It is not possible from the sentence to know, what Duo means with it.
Feed the body or feed the mind?
The "correct solution "is given in the nominative, but the translation here is in the accusative; my translation in the accusative was said "wrong". It seems that DL has a problem here.
What exactly was your translation if I may?