"What books do you read?"
Translation:Jakie książki czytasz?
You don't know whether the speaker is speaking to one person or more than one person if it's not deduced from context. Report it if it's marking either czytasz or czytacie wrong.
With a question like this, though, I imagine I am asking one person, czytasz, because I am expecting an answer to the question. If I am asking a group of people this question, I wouldn't expect them to all start telling me their answer at the same time.
As with all these examples, the English construction using just "what..." instead of "what kind of, which kind of, what type of, what sort of" makes it ambiguous, because that same construction could also simply mean "which books do you read" (of the selection in front of us). People may actually say "What books do you read", but without context, using either "jakie" or "które" is valid.
This construction "What books do you read?" means exactly "Jakie książki czytasz?" and not które. We are certainly not talking about which 2 books from the ones on the table you are reading again and again. You don't need to add "what (kind of books)", because the meaning of brackets is implied, as long as you use present simple and what.
In a very strict sense of grammar, I agree that your reply is the best way to teach the difference, but this course also uses colloquial forms. Coming from the Polish, "czytasz" can be read as "do you read" or "are you reading". I think it would be hard to argue that "What books are you reading" always means "what kind of books are you reading" - I'm sure it is commonly used to mean "Which books are you reading" in colloquial speech.
"Use “what” when there are lots of possible options or when you don’t know how many options there are". Seems to work perfectly fine for this sentence...